Vice Admiral Bertram Ramsey wakes from a dream about the deaths of thousands of young men he has sent to their deaths at sea, dragged into the boiling waves by skeletal hands. He settles back into his chauffeur driven car, patting his briefcase for comfort. Fog is rolling in off the English Channel, obscuring Dover Castle. Beneath the castle a network of tunnels house the operations centre for the British naval war effort. Ratings Fossbrook and Mansell are guarding the entrance when Commodore Jarman approaches the gates to allow Ramsay’s car through. Jarman and Ramsey disappear into the underground complex. Fossbrook is startled when the sky appears to rip in half. If only he had known it he is looking into the bright throat of the time vortex. Two objects race towards Fossbrook and into 1940.

In the vortex is the TARDIs, alarms blaring, sparks cascading on the Doctor and Donna. The Doctor says there is an instability caused by the ship they are following. The TARDIS careers to the left and out of the vortex. All the lights go out.

A grey, shabby room containing maps and a circular plotting table is Ramsay and Jarman’s destination. Ramsey offers his condolences to Jarman for the death of his son. Jarman enquires about the recent meeting with the Prime Minister. Ramsey addresses the assembled men in the room, announcing that they are going to route the enemy. He says that the British Expeditionary Force has withdrawn to Dunkirk, nearly half a million men. They have six days to get the men off the beach. Fossbrook bursts in demanding that Jarman comes outside to see what he has seen, thinking it is a German weapon, but when Jarman and Ramsey get there the spectacular rip in time has gone. Just then a huge noise in the sky knocks them from their feet and smashes nearby windows. A crystalline sphere screams across the sky and impacts into the sea, followed by a tiny blue box. Ramsey recognises that neither is a German weapon and orders an investigation.

The Doctor looks at the scanners, recognizing that they are in 1940. The TARDIS plunges towards the sea, its doors inexplicably swinging open. A howling gale roars round the console.

The Doctor spits out sea water and watches the TARDIS floating beside him. Donna breaks the surface and the Doctor pulls her towards the TARDIS. The Doctor thinks all of this is brilliant. He confesses to having lost the alien vessel. A naval patrol boat approaches through the fog. The Doctor, Donna and the TARDIS are loaded into the boat. Donna wonders if they are under arrest

In the mud of the sea bed the sphere pulses with light. It cracks open and a clawed hand emerges. The creature travels at high speed, detecting something large and metal in the water. It approaches, sensing food and safety.

Ramsey lays his briefing notes on the table wondering how he can justify risking civilian lives in the Dunkirk rescue. A rating tells him that two survivors are being brought in from the crash at sea. He begins the briefing on Operation Dynamo, telling how civilian fishing vessels, yachts and pleasure craft will be used to get soldiers off Dunkirk beach. Then he changes his mind, saying that morale will be destroyed if civilians are killed in the operation. Jarman objects but Ramsey tosses his orders aside.

The creature breaks into a German U-boat and slaughters the men inside. It barely has time to send a mayday before all the men are dead.

Praying in the chapel, Ramsey is disturbed by Chaplain Clayton. They greet each other warmly. Ramsay voices his despair at sending so many young men to their deaths. Meanwhile the Doctor and Donna are led by Fossbrook along the quay side. Donna strikes up a conversation with the sailor, causing the Doctor to groan at her flirting. He reminds her that they need to find the creature quickly. They are led into the base and through the low tunnels. The Doctor abruptly leads the sailor away from their path and into the operations room. He strides in, delightedly greeting Ramsey by name and saying how much he has wanted to meet him. He tells Donna that Ramsey is a genius, renowned for saving thousands of men’s lives. He produces his psychic paper, claiming to be part of a scientific advisor for the Ministry of Defence’s covert forces and introduces himself as The Doctor. Jarman looks at the psychic paper, showing the name Doctor John Smith. Ramsey wonders if they are investigating the ‘shooting star’. Donna agrees and says they are ‘working for the X Files.’

The Doctor urgently insists that there is great danger in the sea but Ramsey is more concerned with the Germans. Jarman says that there has been a mayday from a German U-boat. The Doctor demands that the submarine is not blown up as Jarman desires; he asks Ramsey to let him investigate. Ramsay is caught in two minds, provoking a virulent diatribe from Jarman about Ramsey’s age and lack of recent combat experience. Ramsey responds angrily and then orders Jarman to give his help to the Doctor and Donna. He offers a boat to the pair. Suddenly the chaplain, Clayton, starts to intone about the end of days and the approaching darkness. The Doctor notes that if the creature has come into contact with people then the chaplain may well be correct.

Jarman is seething with fury. He heads for the communications office, planning to have the Doctor removed. He tells Rating Sharpington to send a set of co-ordinates to the Germans telling them the location of the War Tunnels. He then tells the rating that, if he is ever asked, the order came from Doctor John Smith.

Fossbrook, Mansell and Lt. Hobbs take a patrol boat out to the drifting German submarine. They have the Doctor and Donna on board with them. They hail the vessel but there is no reply. The Doctor tells the sailors to stay on board while he takes Donna onto the submarine. Inside, the U-boat is dark. They find canisters of Zyklon B. A dehydrated corpse lies nearby, drained of its blood. The Doctor comments that the breeding cycle has begun. They find more corpses and the bony carapace of the creature itself. It is dead. The Doctor recognises it as a member of a peaceful species and says it can only have gone on a murderous rampage if it was the host to something deadly. One of the German sailors moves and the Doctor finds a weak pulse. The sailor moans that he is not what he appears to be.

They take Engel back in the patrol boat. He hears Donna’s voice telling him that there is nothing to fear. The Doctor radios the base but gets Jarman who refuses to have a Nazi in his base. The Doctor decides to carry on as he intended. Meanwhile, Jarman has investigated the Doctor and Donna by calling the Ministry of Defence.

Donna and Fossbrook have a friendly conversation, during which she denies that she is having a relationship with the Doctor. They kiss briefly, which Donna wants, but then she backs off and tells him that she will be leaving soon. Inside the tunnels Engels is taken on a trolley to the sick bay while the Doctor tries to talk to whoever is inside. He offers to take the creature home. Engels suddenly sneers and announces that “We are not who we say we are,” in a cruel voice. He asks why the Doctor has followed him. Engels suddenly becomes comatose. Mansell leaves nervously and Donna follows him out. Mansell blurts out that he is only 15, wondering how Engels knew.

The Doctor tries to get into his mind but is interrupted when Jarman arrives, pointing his pistol. The Doctor insists that he is treating the German and then he will turn him over to Jarman. They notice that Donna is missing. Jarman leads the Doctor away at gunpoint. At the operations room Jarman shows Ramsey that the Doctor is not part of the Ministry of Defence. The Doctor takes a deep breath and starts to explain the truth; he is tracking an alien creature that threatens the whole world. In the sick bay Clayton comes to give the last rites to Engels. Engels stands up and tells the blind priest that there is nothing to fear. As the priest prays Engels opens his mouth and allows a tiny black creature out of his mouth and onto the back of the priest’s neck. Just then, Fossbrook walks in.

The Doctor pleads with Ramsey who demands absolute proof. The Doctor uses his sonic screwdriver to shatter a teacup that Jarman is using. This still does not convince him so the Doctor shows him the inside of the TARDIS. Ramsey is astonished as the Doctor explains that the TARDIS and he are both alien. Ramsey bemoans the fact that he has always looked to the past, never the future. He wonders if he has the strength to help in the war effort but the Doctor reminds him how valuable he is.

Donna and Mansell enter the sick bay. Fossbrook is climbing to his feet claiming that Clayton killed Engels and then knocked him out. Mansell races after Clayton. A sudden alarm blares out - an air-raid siren. The sound of Luftwaffe bombers can be heard even in the tunnels below. There is a massive explosion above. Sharpington comes in and speaks quietly to Jarman who announces to Ramsey that the Doctor sent their position to the Germans. The Doctor claps slowly, congratulating Jarman. Ramsey sighs, telling the Doctor that he will have to be locked up. The Doctor orders them all to get down as a wall of flame fills the room and a burning rafter falls onto them. The tunnel entrance collapses, sealing them in. The Doctor pulls a beam off Ramsey and scans the old man with his screwdriver. He uses a different setting to regularize Ramsey’s heart palpitations, saving his life. Jarman shrieks that the Doctor was trying to kill Ramsey and uses his gun to club the Doctor insensible. Ramsey climbs to his feet, knowing that he trusts the Doctor more than he does Jarman.

Fossbrook tends to the injured Donna as Mansell returns from the plant room. He says that is where Clayton went so Donna and Fossbrook head that way while Mansell goes to get the Doctor. They follow the priest, amazed that he can see again. Clayton stops and whispers savagely that he is a Nemonite. Fossbrook shines his torch and sees a huge black leech with ferocious teeth clinging onto Clayton’s back. A thick black liquid full of thousands of tiny creatures pours from Clayton’s mouth into an open water tank. Donna tells Fossbrook that he is breeding.

Jarman orders some ratings to take the Doctor to the cells. He is thrown into the corner of the cell where Jarman plans to use three of the ratings to carry out a summary execution. He asks if the Doctor has any last words before telling the ratings to ‘Take aim…Fire!’

The Nemonite spawn swim through the water pipes of the base and burst out of taps, fixing their teeth into any men that they find. They begin to feed. Mansell runs for his life.

Nemonites fall onto the firing squad before they can pull their triggers. Jarman tries to kill the Doctor with a pistol but Ramsey bursts through the door and throws off his aim. A Nemonite slithers towards Ramsey and Jarman shoots it. The Doctor tries to stop him but too late. The Doctor tells them to regroup in the Operations room.

Clayton offers a handful of Nemonites to Donna. They slither towards her and Fossbrook. Fossbrook unscrews a drum of fuel and spills it across the floor. He fires into the puddle, igniting the fuel and roasting the creatures. Clayton pursues them through the wall of fire. They run past infected ratings writhing on the floor that also rise up and start to chase them.

The Doctor steps over Engels body. Jarman points to a shambling, possessed, rating, followed by many more. Ramsey demands an explanation. The Doctor tells him how the Nemonites lay their spawn in a host body. Clayton chuckles and says how the world will soon belong to the Nemonites. The ratings approach. They chitter softly, using echo location. This gives the Doctor an idea and he starts to make a lot of noise. The ratings turn towards him. The Doctor tells Ramsey to be quiet and the ratings pass by him. The Doctor tells Ramsey to order every siren and klaxon to be sounded.

Hobbs and Sharpington lie injured in the Operations room. The phone rings. Mansell creeps down a corridor nearby. He enters the room and receives Ramsey’s orders.

The Doctor orders the Nemonites to leave, giving them one warning. Clayton says that his species is almost dead. He is their salvation.

When the sirens begin the ratings begin to stagger, out of control. Mansell helps his injured colleagues as Ramsey and the Doctor enter. They notice that Jarman has fled. The Doctor orders everyone present not to drink the water. Then the lights go off. The Doctor says that the Nemonites have shut down the generators because they can see in the dark. Donna and Fossbrook burst in ahead of a shuffling group of ratings. They slam and lock the door.

The Doctor is thinking about it being May 26th 1940. He says it is history in the making and this is the day Ramsey saved hundreds of thousands of lives in Operation Dynamo. Ramsey wonders how the Doctor can know this and says he has cancelled the operation. The Doctor tells him that such a course will lose the war. Ramsey concedes that this is right but says he needs power and communications. They order Fossbrook to run down to the docks and run back with a cable from one of the ship’s generators. Hobbs suggests the German submarine. Fossbrook turns to Donna and asks her to help.

Thick fog swirls across the cliff top as Donna and Fossbrook lower a cable to the ground beneath. Fossbrook climbs down easily, Donna with les grace.

Jarman huddles on a bed in Ramsey’s quarters, thinking how he will avenge himself on Smith and Ramsey. The recent deaths of his wife and son have left him with nothing to lose. He dresses himself in one of Ramsey’s Vice-Admiral uniforms and salutes himself in the mirror.

The Doctor plans to climb through the ventilators into the sickbay beneath his feet. Ramsey asks if there is any chance his infected men can survive. The Doctor says no, sadly. Ramsey is relieved because he knows they will all have to be killed. They drop a cable down the ventilator shafts and the Doctor descends. Ramsey gives him directions for finding the sick bay before turning back to Hobbs who is rifling through papers. He shows him a sheet that says there are thirteen canisters of HCN (hydrogen cyanide) in stores, brought off the U-boat. He suggests pouring it into the water supply. Ramsey turns to Mansell and gives his orders.

Mansell shines his torch around the store room. He finds the HCN and tilts a drum into the water tank. He wonders what to do next.

Donna hauls the cable into the conning tower of the submarine. Fossbrook takes it from her and goes into the dark insides of the submarine. She follows him down and finds Fossbrook struggling to read German instructions for wiring the cable to the generator. Donna laughs because the TARDIS is translating the labels for her. She reads them out to him. They quickly complete their task.

The main door of the operations room judders and a metal grille in the wall rattles. Ramsey and his men open fire, killing ratings that crawl in. Sharpington is dragged into a melee and bitten by a Nemonite. Ramsey drives off the majority with a grenade. The corridors outside are quiet and Ramsey and Hobbs are almost out of ammunition. Sharpington turns towards them, drooling black slime. He hisses and lurches away from them into the corridor. Ramsey realises that the rating is heading to the submarine. They chase after him. Ramsey and Hobbs chase outside as Sharpington drops over the cliff and heads towards the U-boat. Ramsey fires a flare towards the U-boat as Donna and Fossbrook emerge at the top of the conning tower. They see Sharpington lurching towards them through the mist and drop inside. They can’t seal the hatch above them because of the cable running out of the submarine. Hobbs recognises this.

Jarman plays ‘Ave Maria on Ramsey’s gramophone. He then leaves the room and heads out into the corridors whistling the tune. Mansell hears him passing by and starts to follow.

Clayton stands in a stairwell, watching dead Nemonites drop from the ruined pipes. He howls in despair. In the sick bay the Doctor hears him. He frowns, sadly and uses his sonic screwdriver to attract Clayton. He looks around at the equipment he has wired together.

Jarman steps into the generator room. He examines the control panel. Mansell watches him, wondering why Jarman is wearing one of Ramsey’s uniforms.

Clayton enters the operating theatre of the sick bay, demanding to know why the Doctor has butchered his children. The Doctor denies any knowledge. He offers to take Clayton home but Clayton says the Earth is too perfect. The Doctor points out that it isn’t the Nemonite’s world but Clayton tells him how the Nemonites were driven underground on their own planet when aliens came to terraform it and then culled. The creature inside Clayton was the lone survivor and possessed one of the aliens and made his escape.

Inside the U-boat Fossbrook and Donna have tied the hatch shut with rope. Donna suggests electrifying the hatch and Fossbrook’s face lights up. They kill Sharpington and race up to view the corpse and the dying Nemonite. Then, together, they connect the power supply to parts of the base. Ramsey calls one of the ships in the fleet out in the English Channel, HMS Brazen, to begin Operation Dynamo.

The power returning gives the Doctor the chance to make his final offer. He tells Clayton that he will use light against them, but Clayton says he has millions of children ready to spawn. The Doctor threatens to switch on the transformer but Clayton says he will be no better than the other murderers of his race. The Doctor says he is sorry before turning on the transformer and making the sick bay blaze with bright light. Clayton freezes and drags the Nemonite from his neck. The creature falls, bubbling behind a cabinet, dying. Clayton’s body explodes into dust.

Jarman turns the generator on and then clambers up into the ventilator shaft. Mansell goes to tell Ramsey what has happened. The Doctor follows him in saying that there are about twenty Nemonites coming; ready to spawn again – probably a million more. He wonders if they can lure the Nemonites onto the U-boat and contain them at sea. Then he could materialise on board and take them elsewhere. The lights flicker as the generator overloads. Ramsey instructs Hobbs and Mansell to aid the Doctor while he himself goes to talk to Jarman.

In the Dynamo room the Generator is running loud and hot. He begins to switch off the unit before it blows up the base.

The two sailors blow open the blocked entrance to the base with dynamite. In the distance they can see the quay side. The Doctor uses infrasound on his screwdriver to mimic a Nemonite distress call.

On the submarine, Donna and Fossbrook hear the explosion. They see Hobbs, Mansell and the Doctor running towards them, pursued by thirty ratings. While Mansell tears off along the quay The Doctor and Hobbs climb up the conning tower and into the submarine. The Doctor asks the sailors to get the submarine out to sea. Donna gives Fossbrook a kiss before the Doctor tells her to leave once the Nemonites are on board. The creatures follow Hobbs as he runs to the front of the submarine with the screwdriver and the Doctor drags Donna behind some crates. Hobbs lures the Nemonites into the depths of the ship. The engines lurch into life and the submarine moves away from the quay with Donna still on board.

Jarman fires a shot at Ramsey’s head, declaring himself the leader of the tunnels. He leaps at Ramsey who says Jarman was promoted to take him out of the way. He points the gun between Ramsey’s eyes.

Mansell pilots a patrol boat alongside the submarine. Inside, the Doctor and Donna step across dead ratings and pools of Nemonites. Amid them, Hobbs lies dead. The Doctor pulls the screwdriver from Hobbs’ hand and Fossbrook has a Nemonite attached to his neck. The Doctor drags Donna up a ladder. They emerge into the air and are about to jump when Fossbrook grabs Donna. He pushes her into the sea and salutes the Doctor. With a terrible scream he wrenches the creature from his neck. Heading below, Fossbrook wades through thousands of Nemonites and prepares the submarine to dive, determined to kill all of the Nemonites. Meanwhile the Nemonites seethe over him as he struggles to keep control.

Jarman holds Ramsey by the overloading generator. He shoots Ramsey in the leg and then leaves. Ramsey drags the main power cable free from the interior of the generator. He goes to the ventilator shaft and climbs up after Jarman. Jarman fires at him, but misses. He climbs down to attack, then slips and plummets down the shaft.

Mansell pulls the Doctor and Donna aboard the patrol boat. The TARDIS stands in the stern. Donna demands that the Doctor gets Fossbrook off the ship. In the TARDIS the column begins to launch and then fails, the effects of the landing in the sea earlier. Donna implores the Doctor to do something. They run to the side of the patrol boat and look at the submerging submarine. The Doctor slams his fist down in frustration.

Using Hobbs, the Nemonites plead for their lives. Fossbrook closes the torpedo tube and fires a torpedo. It explodes and the flames roar back through the submarine, incinerating everything within. On the surface, Donna cries. The wreckage of the U-boat tumbles towards the seabed.

Donna sits in the stern of the stern, holding a letter that Fossbrook gave her for his mother. They see the flotilla of hundreds of civilian craft heading for the beaches of Dunkirk. The Doctor sees Donna’s tears over the death of Fossbrook and holds her close.

As they land, Ramsey waits for news, and then accompanies them back to the base. He tells Mansell that he knows he is not old enough to have joined up, but says he did the same himself.