Original subtitles

(explosion blasts)

Bullseye.

(dramatic music)

Achtung! Achtung!

(dramatic music)

Dive! Dive!

Why the hell have you dived?

(klaxon blares)

(explosion blasts)

Britain may have dealt with Doenitz's aces.

No way back to Germany this time.

But his U-boats keep coming.

Fire!

(torpedo whooshes)

New orders for Group Parkenschlag.

(explosion blasts)

Attack at will in all coastal areas.

They sink ships faster

than Britain can build them.

(explosion blasts)

If we keep losing ships at this rate,

the country will be down to just a few weeks of supplies.

People will be starving to death.

Happy hunting.

The Royal Navy turned to retired war gamer,

Commander Gilbert Roberts,

(dramatic music)

and a group of young women.

(dramatic music)

Sir.

Ladies, if we lose the Battle of the Atlantic,

we lose the war.

Our job is to find a way to sink the U-boats.

That is what we will do here.

Sir, the board is set.

By recreating battles and playing through scenarios

in a war game.

(explosion blasts)

Is that Doenitz's master tactic?

We need to work out how to destroy them.

(dramatic music)

(morse code beeping SOS)

(indistinct radio report)

(people talking indistinctly)

Gilbert Roberts reports to Western Approaches Command,

the Royal Navy's center of operations

for North Atlantic convoys in Liverpool, England.

Gilbert Roberts could be doing so much more at this point.

Here's a man who's had two years training in war games.

War gaming allows you to understand how tactics work,

and it allows you to explore new tactics.

Churchill couldn't be less pleased

with how the Battle the Atlantic is turning out,

the fact that they're losing hundreds of thousands of tons

of merchant shipping to U-boats.

Enter.

What Churchill has to do,

not just stay in the war, but win it.

You can only win it if you're gonna win

the Battle at the Atlantic.

Sir, Commander Roberts.

Thank you. That will be all.

(door slams)

Commander Roberts, retired.

I thought London was sending me a captain.

No, sir.

Sector commander, Portsmouth Naval Anti-Invasion Battalion.

The Home Guard?

(sighs) Instructions from the Admiralty say

that I'm to give you the top floor

of the exchange building.

For?

A school, sir.

School?

A tactical school.

The prime minister has asked me personally

to analyze U-boat attacks,

decipher their tactics

and develop counter tactics to sink them.

These would then be taught to escort commanders

at the school.

We already have a counter-tactic, Roberts.

Buttercup, developed by our own Commander Walker,

a man who has actually sunk U-boats.

Have you?

No, sir.

And I'd very much like to begin with Buttercup.

By analyzing Commander Walker's actions,

I can then play through other scenarios in a war game

and identify any tactical weakness that we-

War game? Tactical weakness?

I'm sorry, but what does a sector commander

of the Home Guard know of tactics?

I spent two years at the Portsmouth Tactical School, sir,

war gaming (coughing)

before retirement.

A school to play games.

And I'll need men to staff the unit too,

men with good analytical minds, good with records.

But mostly good at playin' the game.

Commander, I'm in the middle of fighting a war.

All my best men are at sea.

I might be able to scrape together a couple

who are unfit for active service.

But if you need more to play your game,

you should speak to London.

Thank you, sir.

Oh, and Roberts, this school,

perhaps you can run occasional courses

for half a dozen reserve officers,

but don't bother me with it.

I'm a busy man.

Dismissed.

Percy Noble doesn't seem to be taking on board

the importance of finding new tactical ways forward.

(Roberts coughing)

Doenitz is a tactical master.

This is a time where it's more crucial than ever

that he listens to Roberts.

Sir.

Secrecy is paramount.

Yes, quite right.

And for the Royal Navy,

help arises from an American tragedy.

After the attack on Pearl Harbor,

the USA declares war on Japan.

Four days later, Germany sides with their Japanese ally

and declares war on the USA.

The Americans have already been

unofficially supporting Britain through the war.

Now they mobilize at full strength.

The idea was to get to Germany, to defeat it first.

But, of course, doing that meant getting supplies

and equipment and now manpower as well

to the British Isles in preparation

for an eventual invasion of Nazi Germany.

But the American involvement created new opportunities

for the German U-boats to attack American shipping.

Doenitz saw this immediately.

The United States Navy should have been prepared

for a submarine campaign,

but the problem they have in early '42

is they don't have enough destroyers

trying to protect merchant shipping.

Most of them have gone to the Pacific to fight the Japanese.

(guns blasting)

This lack of protection presents Doenitz

with a golden opportunity.

He measures his U-boat captains' success

by the number of ships they send to the bottom.

Admiral staff's officer.

Orders for Group Parkenschlag.

Proceed to naval square CA28.

New York.

Yes, Herr Admiral.

They knew that the shipping

would concentrate off New York,

off Cape Hatteras, off Cape Cod.

American involvement in the war gave the U-boats

a lot of new targets off the American coast.

Herr Admiral.

(dramatic music)

Have the Americans learned nothing from the British?

(scoffs) Very heavy traffic,

mainly north-south.

Very many tankers,

occasional neutrals,

no surveillance.

(aide chuckles)

They're not even trying to protect their ships.

The United States Navy don't have the escort vessels

they need to establish a system of convoy.

So, they decide not to do that.

There's a marvelous directive that comes

from the Office of War Information.

Quite simply, the Germans can sink

as many ships as they like.

It really doesn't matter because American industry

is just gonna bury them in new shipping.

All the more prey for the wolves to feed on.

(dramatic music)

New orders for Group Parkenschlag.

Attack at will in all coastal areas.

(dramatic music)

(explosion blasts)

January of 1942, they sank 41 ships, 250,000 tons,

in just two weeks.

(explosion blasts)

And a lot of those sinkings were just off the coast

of the United States, literally within sight

of American home territories.

(explosion blasts)

German success is so profound

that it becomes known as the "second happy time"

for the U-boat fleet.

With the Americans being destroyed in such numbers,

it's more and more important

that the convoys that do survive are protected,

that the Royal Navy gets their ships across.

(submarine rumbles) (dramatic music)

The scale of the task is enormous.

Every available man is put to work,

but there aren't enough.

Without women, we realize we can't win the war.

Now that is a whole new ball game

for men to get their heads around

and for young women to engage with.

The Royal Navy are forced to rely more and more

on Vera Laughton Mathews' Wrens.

I do appreciate having your Wrens here.

Certainly brighten the place up.

They can do more than that.

Much more.

To begin with, the Wrens have just nine roles,

and they are fairly humdrum.

If you're lucky, you're a typist, all right, or a driver.

Those minimal opportunities expand to over 125 roles.

One of those are as codebreakers

at the highly-secretive Bletchley Park,

deciphering radio messages between Doenitz and his U-boats,

providing vital intelligence

on when to expect U-boats attacks.

But despite the best efforts of the men and women

of the Royal Navy, they still haven't deciphered

the enemy's tactics.

(dramatic music)

Even though Roberts initially gets

a very frosty reception.

He is delighted to be back.

He had been cast out of the navy

due to having TB just four years earlier.

(Roberts coughing)

This is a man who had spent quite a lot of time

in the interwar period thinking about these problems.

He can use his skills. He can contribute to naval success.

And if possible, he can contribute to victory.

As if to undermine Roberts' position even further,

he isn't put in the main headquarters.

His offices are in a different building

which hasn't even been finished.

It's still under construction and it's been bombed.

What's this?

(debris clatters)

A gift from Hermann Goering.

Damage from an incendiary bomb.

It needs to go. Arrange for it to be cleared.

Yes, sir.

(Roberts coughing)

(Roberts gasping)

I'll need battle reports.

Anything detailing a successful you U-boat kill,

including convoy HG 76,

Commander Walker's Buttercup triumph.

Yes, sir.

He needs as much information as he can get

about what's really facing commanders

when they're out in combat

to figure out what the U-boats are doing

and how to defeat them.

I also need supplies to build the game board.

(paper shuffling)

Canvas sheets, roll of linoleum.

Chalk, string, paint, and model ships, yes?

Yes, sir.

Very good.

These requests raise eyebrows at Western Approaches.

I mean, here is this newcomer who wants to play games

while battle rages in the Atlantic.

Anyway, sir, all this tactics and war gaming

might be for naught.

We already captured that Otto Kretschmer

and they already started interrogating him.

Surely, he'll spill the beans

and give up all of Doenitz's secret tactics.

Surely.

(dramatic music) (waves crashing)

From 1939 to 1941, Kretschmer unleashed a reign of terror

against Allied shipping.

A hunter's moon, just as I like it.

Prepare for attack!

Yes, sir.

Fire bow torpedo.

Sinking 47 ships across the North Atlantic,

over 273,000 tons.

More than any other U-boat ace.

That's as good as it gets.

The human toll is 830 lives.

For his service to the Third Reich,

Hitler awarded him the Knights Cross of the Iron Cross

with oak leaves and swords.

Danke schoen.

(heels click)

At the time, Nazi Germany's highest naval award.

Achtung! Achtung!

(guns blast)

Dive! Dive!

(klaxon blares repeatedly) Why the hell have you dived?

Are you sure we couldn't have gotten away on the surface?

But in March, 1941, his luck ran out.

(explosion blasts)

(metal creaking)

Sir, we've lost all thrust in the propellers.

Blow the ballast tanks, everything we've got.

We have to lose depth.

(thunder cracks)

Prepare scuttling charges

and destroy all documents and ciphers.

(dramatic music)

No way back to Germany this time.

(dramatic music)

(footsteps treading)

(chair clatters)

Otto Kretschmer is this extraordinary U-boat ace.

He's caught alive. That is such a win for the Allies.

The MI9 have got hold of him.

MI9 is a unit established to interrogate prisoners of war.

It's like, if you like, a latter day Guantanamo Bay.

"Here's your chance, boys. Drill into him.

Download his microchip of knowledge,

and you are gonna turn the tide of this war."

(dramatic music)

No, thanks.

You speak English.

Would you rather we spoke German?

No, English is good.

I might as well get used to it now.

Quite an impressive tonnage tally, Commander Kretschmer.

Must make you very popular back home.

The "Tonnage King," isn't that what Goebbels calls you?

It's just propaganda.

I make good copy for the Nazi papers.

My picture gets more women waving flags,

more men signing up to the U-boat corps.

What for?

To sink more ships for Hitler?

Sorry if I don't seem too, uh, enthusiastic.

I grew weary of this war long ago.

You won't miss sinking ships?

I have sunk enough ships.

The thrill of the hunt left me long ago too.

And how do you hunt?

You've sunk more ships than anyone.

How do you do it?

What are your tactics?

A U-boat captain, I mean a real captain,

has to be able to take advantage of whatever

the passing moment offers.

You never know when you'll stumble upon a convoy.

A real captain is born with the intuition

to know when to attack and how to press home your advantage.

Smashing.

Do they get the key to Otto Kretschmer's knowledge?

Do they find a way in?

Do they? Hell, as like.

This guy ain't the top U- boats captain for no good reason.

Kretschmer reveals very little to his captors.

Doenitz's U-boat tactics remain a mystery.

(dramatic music)

At Western Approaches Command,

Roberts tries to decipher them.

Sir, the report you requested.

Ah.

Convoy HG 76.

Commander Walker's famous Buttercup tactic.

Thank you.

Sir.

The report of proceedings was a standard report

that was written by anybody in the navy

who had encountered the enemy.

It explains what the enemy has done.

But it also explains what the commander

from the Allied side has done.

Buttercup astern!

HG 76 is important because Walker

had some success doing it.

He wants to understand why Walker was successful.

(glasses clink)

Sir.

What has he thought up?

Why did he behave the way he did?

But if you're looking at a report, any kind of report,

they're very, very dry.

They don't really tell you anything

about the human side of decisions that are being made.

(door knocks twice)

Enter.

(latch clicks)

Sir, Commander Roberts.

What is it, Roberts? (door slams)

Sir, I'd like permission to speak

with Commander Walker about convoy HG 76.

Don't you have his report of proceedings?

Yes, sir. I do.

And you've read it?

Yes, sir.

But to truly understand how Buttercup works,

I need to speak with the commander.

What assumptions did he make?

Why deploy his escorts where he did?

Roberts, Buttercup works.

It's a sound tactic, which is why I ordered

for it to be put into group operational instructions.

I'm aware, sir.

And now our escorts are being fitted

with a new piece of technology.

Ship-mounted radar will be able to spot U-boats

on the surface miles way.

That would be a great help, sir, undoubtedly.

But to complete my work,

I will still need to speak with Commander Walker.

(suspenseful music)

(sighs) Very well.

When Walker gets back from sea,

if he can spare the time, you may.

Thank you, sir.

Now, if you don't mind.

(door thumps)

And Noble has another reason to be confident.

The work of the Wrens is turning

Western Approaches Command into an intelligence hub.

Up and down the country,

Wrens working in radio listening sites,

known as Y-stations, are tracking U-boat positions.

My first station was on the Yorkshire coast,

not very far from Scarborough

where there was a very big listening center.

By comparing the bearing

of the U-boat's radio signal with another Y-station,

the U-boat's position can be triangulated.

U-boats, of course, very seldom spoke.

But now and then, one would come up

and would do a signal to its base or to other U-boats.

When they did, there was a very, very tense atmosphere

because there was such a menace to our convoys and ships.

I was there when a U-boat came up

while they got the signal from the U-boat down,

and particularly, the position of the U-boat.

Because they could then report this back to Admiralty

and they might be able to take action.

Very good.

That intelligence arrives pretty pronto

via radio to Western Approaches

telling you the real-time positions of U-boat flotillas.

So, you plot them.

You mark them on your giant map on the wall.

And Western Approaches are also benefiting

from long-range, aerial photo reconnaissance.

A lot of the analysis of these photographs

was done by women.

They wanted to see, for example,

the U-boat pens along the French coast.

By looking at these sorts of images,

you had more of an idea of when they were going

to leave the pens.

When did they get back?

How long were they out at sea, and what was their direction?

It means Western Approaches are much more aware

of when to expect attacks.

Excellent. Thank you.

That's good work.

But they still can't work out

how the U-boats are attacking the convoys.

Sir, this is the room.

Candidates will be with you shortly.

Thank you.

Yes, sir.

Before he can begin his work,

Roberts clearly needs a team.

He needs people who are quote-unquote whip sharp.

He needs the finest brains.

Boys who are gonna knock the tactics back at the enemy.

(door latch clicks)

(door clatters)

Please, take a seat.

(candidate sighs)

Lieutenant Commander Cousins, DSC, sir.

DSC?

Distinguished Service Cross, sir,

awarded for gallantry against the enemy.

I see.

What experience do you have of tactics?

Tactics?

Uh, we smash the U-boats with depth charges.

That's all we need.

We hound them out of the Atlantic.

(dramatic music)

I see.

Thank you.

Thank you.

(door latch clicks)

(door clatters)

(door latch clicks)

(door clatters)

Oh, yeah, ooh, there.

Lieutenant-Commander Hyam, sir.

Your experience of tactics?

Sir, I've served on both submarines and ships.

And tactics?

My ship sunk.

I had to be hauled outta the water on a rope. (laughing)

(dramatic music)

Dismissed.

Doenitz is a genius.

He needs men who are able to think mathematically,

tactically, very clear-headed, clear-minded.

These are clearly not the sort of men

that he's going to need to decipher Doenitz tactics.

(door latch clicks)

(door clatters)

(briefcase thuds)

Commander Roberts, Vera Laughton Mathews,

director of the Women's Royal Naval Service.

The Wrens?

(papers shuffle)

(curtains sliding)

I hear you're in need of staff.

I can meet all your requirements.

I've been recruiting the people you need for three years.

Wrens? I need naval officers.

And how's that working for you, hmm?

Wren Drake, please enter.

(door latch clicks)

(door clatters)

Please take a seat.

Elizabeth Drake.

The daughter of an insurance actuary, yes?

Yes, ma'am.

Measuring risk must run in your family, yes?

Yes, ma'am.

Thank you, Drake. That'll be all.

Yes, ma'am.

Sounds essential for your tactical game, commander.

Next!

Wren Nancy Wales, sir

Wren Wales, what experience do you have of tactics?

Well, sir,

I captained the Yorkshire hockey team for years.

An expert in team tactics.

Thank you, Wales.

Janet Okell, I hear you are a formidable bridge player.

Well, yes, ma'am.

And what makes for a formidable bridge player?

Well, readin' your opponents is important, ma'am.

Reading opponents and scenarios strategically.

Ma'am, what is this all about?

Thank you, Okell. That'll be all.

And finally, a real gem.

Wren Laidlaw, please enter.

(door latch clicks)

(door clatters)

(dramatic music)

Good afternoon, ma'am.

Sir.

Wren Jean Laidlaw, accountant.

Yes, sir.

I haven't met a woman accountant.

I'm one of the first to qualify

in the United Kingdom, sir.

I see.

Keen mind for numbers, I suppose?

Yes, sir. Very keen.

Just what your game needs to calculate those moves.

I suppose.

Indeed.

Thank you, Laidlaw.

That'll be all.

Yes ma'am.

Sir.

(dramatic music)

(door clatters)

Right.

All of them, commander.

And they all bring a special set of skills to the table.

Exactly what the game needs.

In the early war period,

almost every nation does not consider

women for these sort of roles.

The Wrens were a risk.

The task these young women are set

is one of the most important in the war.

If the Battle of the Atlantic is lost, Britain has lost.

And if Britain has lost,

the Western Allies will probably lose the war.

(whimsical music)

In Liverpool, Gilbert Roberts and the Wrens

begin the next phase of their mission.

What knot is that?

It's a bowline, sir.

Where'd you learn it?

The Sea Rangers.

The simplest, most resilient.

(whimsical music)

Very good.

Men, rig the canvas sheets around the frames like these.

Yes, sir.

Sir, your linoleum, string, paint, chalk,

and model wooden ships.

Excellent.

Any news on the great Otto Kretschmer's interrogation.

Did he spill their U-boat secrets?

Of course he didn't.

That would be far too easy.

It's up to us to decode them.

And the first step for that is to build the board

for the war game.

The board, sir?

The board.

(rug shuffles and clatters)

(Roberts coughs)

The board

Sir, are you all right?

The board.

This is the thing which is going to win

the Battle of the Atlantic.

I mean, the idea is preposterous.

Oh, and sir, Commander Walker's just returned from sea.

He's waiting in your office.

Well, why didn't you say!

(dramatic music)

Buttercup.

Wren Laidlaw.

Yes, sir.

Wait 10 minutes and join me with Commander Walker.

Yes, sir.

(dramatic music)

Roberts wants to understand how Buttercup works.

Commander Walker.

The chance to sit down with Walker

and actually try and get inside his mind,

this is a crucial moment.

What's this about, Roberts.

You want a debriefing on Buttercup to help play a game.

A war game to play through scenarios,

decipher Doenitz's tactics

and learn how to sink his U-boats.

Something I've actually done using Buttercup.

Which is why I wanted to speak with you.

Convoy HG 76.

32 merchant ships, sailing homeward from Gibraltar.

19th of December, HMS Stanley is torpedoed,

and you order Buttercup.

What happens then?

(explosion blasts)

Buttercup astern.

Well, I've drilled all my escort commanders

to go to battle stations on the order "Buttercup."

They, uh, turn away from the-

Ah, Laidlaw.

Please, take a seat.

Commander Walker, this is Wren Laidlaw,

a member of my staff.

Laidlaw is a, is an accountant by trade.

I've never met a woman accountant.

Laidlaw is one of the first in the country to qualify.

So, now you have.

Ah, the Stanley was hit.

We can't be sure exactly where the U-boat fired from.

But they fire from outside the convoy in.

Once a ship is hit,

they escape by motoring along the surface.

When we order Buttercup,

all escort ships turn outward away from the convoy,

full speed, chase down the fleeing sub.

Sir, how can you see the U-boats?

The attack was at night, wasn't it?

We fire star shells, flares that light up the sea.

Uh, we search for 20 minutes.

We spot a U-boat.

Uh, well, we destroy them.

(dramatic music) (waves crashing)

Find that damned hearse and sink it.

Buttercup starboard.

(flares pop and sizzle)

Come into our little trap.

(water splashing) (dramatic music)

Prepare to ram.

(dramatic music)

(ship crashes)

Buttercup has prevailed.

Dearest Buttercup.

And it worked?

My dear, we only lost two convoy ships.

I personally sank two U-boats

and three more were hit by other forces under my command.

Sir, HG 76 had 16 escort ships

protecting 32 merchant vessels.

But is that a heavy escort?

Well, uh, more than most North Atlantic convoys.

But sailing out of Gibraltar, if you don't go in heavy,

then you're asking for trouble.

And how many escort ships were lost?

Two escorts.

Including the aircraft carrier HMS Audacity.

(dramatic music)

Buttercup works.

We sank five U-boats.

Admiral Noble even asked me to write it up

for the escort group operational instructions.

Yes.

So I understand.

Thank you.

Everything they tried previously just failed.

And so this was really the first glimmer of success.

I mean, it was a success.

Walker manages to sink two U-boats himself,

and three others were sunk in this whole operation.

But it was very, very costly.

(dramatic music)

(light music)

Everything's in place.

Roberts has got his game.

He's got his bits in order to make his game work.

He's got detailed battle plans,

so he can work out what the U-boats have been doing.

And to help him, he's got his team of brilliant minds.

These young, dependable women from all sorts of backgrounds.

(footsteps thumping)

So smart.

Vera Laughton Mathews must be incredibly proud.

She's handpicked these young women herself,

and these women are not being asked to do

rather menial tasks of answering phones or driving cars.

This was taking the fight to the enemy

and spearheading a mission to save lives at sea.

This was using gray matter,

women's gray matter.

Wrens, test make a start.

Welcome to the Western Approaches Tactical Unit.

WATU, for short.

Commander Roberts, (door slams)

Janet Okell.

Sir, I'm so sorry I'm late.

I, I, I took a wrong turn and then another and another,

and I couldn't find my way back

It's all right.

So sorry, sir.

And I don't have a uniform yet.

That's fine. I'm glad you could join us.

Uniforms are here.

Please, collect yours.

This is Chief Yeoman Rainer.

He worked with me at Portsmouth.

He knows the game inside out.

Sir, this isn't a Wren's uniform.

Well, you'll be working a lot on the floor.

Sir?

Measuring on your hands knees and so.

Sir.

For this work, the standard skirt

isn't exactly appropriate.

We are wearing the trousers now.

(Wrens giggle)

Indeed.

Ladies, if we lose the Battle of the Atlantic,

we lose the war.

Our job is to find a way to sink the U-boats.

But before we can do this,

we need to decipher Doenitz's tactics.

That is what we will do here

by playing through scenarios and recreating battles

in a war game.

This is our board.

A block for the convoy,

model ships for the escorts.

The players are divided into U-boat captains

and escort commanders.

Now, the escort commanders stand behind these screens

and view the board through these lenses.

Wren Drake, would you?

How much of the board can you see?

Sir, I can't see anything.

Ah, yes. Bit of a problem.

(stool thumps)

How about now? What can you see?

Not much, just a few models.

Just like at sea.

Exactly.

It mimics limited visibility,

just like a convoy stretched for miles across the ocean.

And the U-boat captain, Commander Roberts.

The U-boat captain sits at the high table.

There isn't a model for the U-boat.

No, the escorts can't see the U-boats.

Just like at sea.

So, how do we plot their position, sir?

Brown chalk.

Drake, can you see the U-boat track?

Not a thing, sir.

Brown on brown.

Thank you, Drake.

An adjudicator hands action events to the players

written on these chits.

U-boats contacts, aircraft sightings, ASDIC pings,

ships torpedoed.

The players are then given two minutes to make their move.

They write down their instructions on these movement chits.

Course, speed, ASDIC sweeps, depth charges dropped.

Then they hand them back in silence

so the opposition doesn't know their intention.

And the plotters move the models.

The aim of the game is to sink the U-boats.

And the battle report we'll start with is convoy HG 76,

Buttercup.

(dramatic music stops)

(bell clangs once) (dramatic music)

Despite the Royal Navy's confidence

that ship-mounted radar and better intelligence

will turn the tide against the U-boats,

they're still sinking ships faster

than Britain can build them.

(explosion rumbles)

More and more U-boats keep coming.

Doenitz developed the idea that 100 U-boats

on active patrol in the North Atlantic

could all but shut down transatlantic shipping.

In April, 1941, Doenitz had only 28 U-boats

hunting at any one time.

By the beginning of 1942, that number has exploded.

How many boats do we have at sea?

88, Herr Admiral.

88?

That's close to the 100 we need to shut down the Atlantic.

What is the monthly tally from Group Parkenschlag?

Parkenschlag, a five U-boat wolf pack.

23 merchant ships sunk for 150,000 tons in the last month.

And they ran out of torpedoes.

But they are just five U-boats.

This is a feeding frenzy.

They are gorging themselves off the coast of America.

(dramatic music) (thunder rumbles)

(equipment rattles)

Fire!

(torpedo whooshes)

(explosion blasts)

The German U-boat crews are having a field day

off the American east coast.

There were so many available targets

that U-boat skippers could pick out the ones

that they thought would be most useful.

This led to a kind of specialization on hunting tankers.

The most valuable prizes because fuel meant war.

(explosion blasts)

Britain's war machine runs on oil,

and very little oil is making it across the Atlantic.

Without fuel, Britain dies.

The war effort stops.

This is a very productive hunting ground.

Let's go for the jugular.

(dramatic music)

(music crescendos and stops)

With everything in place, Roberts and his team begin.

And the first thing that they're gonna do

is look at Buttercup and the HG 76 convoy engagement.

The first time that they actually set up

for the war games, they don't play a game itself.

This exercise is really for Roberts

to get all of the ducks in a row,

all the pieces on the board,

to figure out what actually happened.

Every single thing they know, step-by-step.

Where did the convoy go?

Where were the escorts?

Where were the U-boats sunk?

It provides Roberts and his team with a bird's eye view.

That is gonna give them a real advantage

in understanding what went right

on the side of the Royal Navy,

but also what the German U-boats themselves

are most likely doing.

Sir, the board is set.

Excellent.

Let's begin.

Convoy HG 76.

32 merchant ships with an escort screen of 16 ships.

The escorts form a protective perimeter.

A merchant ship is hit.

What position?

The merchant ship, sir, not the escort, HMS Stanley?

No, the merchant ships are our lifeblood.

I want to see how they're targeted.

The SS Annavore, convoy position, five three.

Fifth column, third ship.

(piece clacks)

Now what?

Commander Walker orders Buttercup.

The escorts turn outwards at full speed

and fire star shells to illuminate the sky.

They speed away from the convoy

to try to spot the U-boat.

Walker manages to sink a U-boat earlier

at the rear of the convoy.

Astern of the convoy.

Astern.

(dramatic music)

Good.

At least one U-boat down.

Proof they attacked from outside the convoy,

like Commander Walker said.

(suspenseful music)

Is something wrong?

The merchant ship is hit in the center.

Can they even hit?

What's the range of a torpedo?

Uh?

Leave the board as is.

I need to make a phone call.

(suspenseful music)

(dramatic music)

Operator.

Flag office, submarines, London.

Connecting you now.

Roberts has an old friend who's based

at Submarine Command.

So, he puts in a call.

To his surprise, it's answered by the commander

of submarines for the Royal Navy.

That's Sir Max Horton.

(telephone rings)

Admiral Horton.

Apologies, sir.

I was hoping to speak with your chief of staff.

Captain McIntyre isn't present.

You're through to the flag officer himself.

Who is this?

Now, Max Horton is a big deal.

He is the head of the Royal Navy submarine arm.

He was a submariner in World War I.

He knows everything there is to know

about submarine warfare.

Sir, i-i-it's Commander Roberts

of the Western Approaches Tactical Unit.

I wanted to speak with Captain McIntyre

on submarine tactics.

I served in subs during the first war. What do you need?

Oh, thank you, sir.

What is the range of a German torpedo?

The G7E, roughly 5,400 yards, but that's the limit.

Effective range, expect to halve that.

Now remember, the ship that was sunk in HG 76,

that ship was in the middle of a convoy.

A convoy is 10,000 yards across.

I mean, how does that make sense?

Commander, are you still there?

Y-yes, sir.

Uh, one more thing, if I may.

During the first war,

would you ever fire from inside the convoy?

Of course.

It's the only way of being accurate.

Inside. (dramatic music)

They're bloody well inside.

Roberts, does this answer your question?

Yes, sir.

More than you can know.

(receiver clatters)

(dramatic music)

Commander Walker, the Royal Navy,

they all believe the U-boats fire from outside the convoy.

But what if they're wrong?

What if they're firing from inside?

The U-boats Walker sank.

It would mean Buttercup is based on a false assumption.

The Royal Navy are relying on luck to protect the convoys.

The navy knew all along,

but they forgot they knew.

Is that Doenitz's master tactic?

(dramatic music)

It's getting late.

Wales, Drake, you're dismissed for the evening.

Get some sleep.

Laidlaw, Okell, Rainer,

I want you to stay.

As long as you need, sir.

We need to work out how the U-boats

are getting inside the convoy.

And find a tactic to destroy them.

(dramatic music)

Mein Fuhrer, I can strike a decisive blow,

but I need more U-boats.

(dramatic music)

U-boat destroyed.

How?

Run the game again. We need to be sure.

Who devised this tactic?

Wren Jean Laidlaw, sir.

And do you have a name for it?

(explosion blasts)

Raspberry.

As in blowing a raspberry at Hitler, sir.

Well, let's hope it works at sea

because now Doenitz its has its 300 U-boats.

He'll be hell bent in using them to turn the Atlantic

into a convoy graveyard.

(music crescendos and declines)

Imagine the carnage we will sow.

(dramatic music)

(music stops)

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