The TARDIS hits an anomaly in the Vortex, and although it subsequently materialises on 20th-century Earth, the Doctor can’t tell exactly where. Ace emerges from the TARDIS immediately, eager to explore, for if the Doctor doesn’t even know where they are, she can’t be walking into one of his master plans. They have materialised in the courtyard of a castle which appears strangely familiar to the Doctor, and as Ace takes a look around he steps back, gets a better perspective and realises where they are -- too late. The sound of running footsteps echoes across the courtyard; floodlights slam on, blinding them; shouts ring out in German; and as the Doctor and Ace try to flee back to the TARDIS, the Doctor is shot in the shoulder. Before Ace can do anything, she and the Doctor are surrounded by German soldiers, led by Feldwebel Kurtz. The sentries saw the TARDIS materialise and gave the alarm immediately. Now the TARDIS belongs to the Nazis, and the Doctor and Ace are prisoners in Oflag 4-C... otherwise known as Colditz.

The other prisoners are turfed out of their beds for roll call, and Hauptmann Schäfer watches wearily as they complain about the break in routine and demand to know what’s going on. Flying Officer Bill Gower keeps order as the prisoners watch Kurtz escorting the new arrivals out of the courtyard, and as they realise that one of the newcomers is a girl -- and a spirited one at that. Ace’s insolence and the jeers of the British prisoners don’t sit well with Kurtz. She gives her real name -- Dorothy McShane -- but still refuses to give Kurtz the respect he feels he deserves, and chats casually with another prisoner, Tim Wilkins, as he passes by on the way to roll call. Furious, Kurtz orders her to strip for delousing, and rips the rucksack from her back when she refuses. The rope ladder and explosives inside the rucksack seem to confirm his suspicions that she came prepared to rescue her British comrades, but the CD-Walkman is just confusing. He confiscates all of her belongings, vowing that German scientists will study the technology, learn its secrets and win the war. Ace is suitably unimpressed by his threats.

Schäfer marches the Doctor to solitary confinment, genuinely apologetic that his prisoner will have to wait for morning until the camp doctor can tend to his injury. However, the Doctor seems to be recovering nicely -- and the guard who’d searched him and found nothing is baffled when the Doctor pulls a set of spoons from his pocket and begins to play them. Schäfer isn’t impressed, and warns the Doctor that the commandant has taken a special interest in the machine which materialised from thin air. The Doctor is under no illusions as to what will happen to him if he refuses to reveal its secrets... but he does realise that Schäfer really is trying to help him, and extracts a promise that Schäfer will see that Ace is taken care of.

Kurtz then arrives to deliver his news about the amazing technology in Ace’s rucksack, but gets upset when he sees the care which Schäfer is showing to the Doctor. As far as he’s concerned, the prisoners should be made to talk; compassion is out of line. But Schäfer understands that what Kurtz is really angry about is the prisoners’ lack of respect. Regulation demands that all new prisoners be stripped and deloused, but Schäfer flatly refuses to let Kurtz force Ace to remove her clothing. Deeply angry, Kurtz chooses not to tell Schäfer about the new technology after all; instead, he returns to Ace’s cell to question her further. She tries hiding behind the door, throwing a blanket over him and fleeing, but the guard outside stops her from getting very far. Attacking a German officer is an offence punishable by death, but Kurtz decides not to shoot her... not this time. But now it appears she owes him a favour.

Morning dawns with sirens summoning the prisoners to roll call -- all but the Doctor, who is still in solitary confinement, recovering from his bullet wound. Or so Schäfer thinks until the chipper Doctor hands him the bullet, which he claims worked itself out of his wound during the night. Schäfer has him taken to the camp doctor anyway, warning him that the commandant has already reported the capture of the TARDIS to the German High Command -- and that an agent of the Gestapo will soon arrive to torture the Doctor’s secrets out of him. But by the time they leave the medical bay, Schäfer and the medic have both suffered something of a shock. The Doctor won’t let Schäfer deny the truth; he places Schäfer’s hands on his chest, forcing him to feel the double heartbeat for himself. Badly shaken, Schäfer nevertheless takes refuge in his duty; he will wait for orders, and do as he’s told.

Ace has been given her own cell, but must take meals and recreation with the other prisoners. Gower introduces himself, and although curious he doesn’t ask too many questions, assuming that she’s here on a top-secret mission. She, like young Tim Wilkins, is a prominenter -- a valuable prisoner -- although Wilkins bitterly claims that he’s done nothing to deserve this status. Ace brings up her escape attempt, and is infuriated when the other prisoners laugh at her, assuming that they’re patronising her because she’s female. Gower tries to let her down gently; although he’s surprised by how much she already knows about Colditz, there are a few harsh realities which she doesn’t yet understand. Colditz isn’t called “the escapers’ prison” because it’s easy to escape from -- rather, this is where they keep officers who have already escaped from other camps and have been recaptured. This is the most secure POW facility in Germany. Nobody has escaped for a year, and every time an attempt fails another route is cut off forever. Escaping from Colditz won’t be as easy as Ace had assumed...

Tim Wilkins finds Ace looking out over the courtyard, pondering her situation. Gower has also lectured him about the futility of escape, but he doesn’t buy it; he knows that escape plans are being made all the time, and is convinced that Gower and his cronies pick and choose their favourites. Tim knows that the war is going badly for Germany, and that when the Allies push forward the Germans will start using their prominenter as hostages. He’s determined to escape before that happens, but he needs help. Ace, touched by his enthusiasm and naivete, admits that her real name is Dorothy -- but tells him to keep calling her McShane.

Kurtz arrives and orders Ace to accompany him back to the Doctor’s cell, where the Doctor is still trying to find out how Klein knows that his ship is called a TARDIS... and that there will be no record of his or Ace’s existence in 1944. She ignores his questions, and as soon as Kurtz arrives she orders him to shoot Ace. It’s an order he’s only too happy to carry out, but at the last moment the Doctor cracks and hands over the TARDIS key. Klein has already ordered Schäfer to meet them at the TARDIS, and she now orders Kurtz to keep an eye on Ace; if he even suspects that the Doctor isn’t co-operating, then he is to kill her. The Doctor advises Ace to play on Kurtz’s paranoia, and accompanies Klein to the TARDIS, which he is forced to open for her and Schäfer. Schäfer is stunned by the size of the interior, and realises that he’s far out of his depth... but he washes his hands of the responsibility and simply leaves to report what he’s seen to the commandant. Klein is contemptuous of his lack of vision, but is willing enough to leave the TARDIS in his hands. She came to Colditz not for the TARDIS, but for the Doctor.

Ace starts in on Kurtz as soon as they’re alone, making much of the fact that Klein won’t let him in on any of her secrets. Kurtz has all but dismantled the CD-Walkman trying to find out how it works, and he’s furious when Ace claims that Klein will now take all of the credit for all his hard work. Nevertheless, he has his orders and must restrain himself as Ace mockingly treats him like a lapdog, running errands for his masters. He escorts her to the mess hall, where she joins Wilkins in a meal of mashed turnip. Tim is still desperate to escape; as far as he’s concerned he shouldn’t be a prominenter at all, as he’s just a war reporter who’s unlucky enough to have a cousin in an important position in the War Office. Gower then joins them and asks Ace if she has any good news he can pass on; obviously her work must remain top-secret, but the lads would appreciate a morale boost. Ace assures him that the war will be over by next year, and Gower, appreciating her effort, finally agrees to let her into the loop. Despite Tim’s nervousness, he agrees to create a distraction, enabling Ace and Gower to slip away and embark on a tour of the prison’s known weak spots. However, Kurtz catches up to them in the prison kitchens, and slaps Ace for trying to get away from him. He can’t do anything more with the other British prisoners as witnesses, but he intends to take her to her cell and... discuss the “favour” she owes him. Furious, Ace pushes him aside, and despite Gower’s protests, Kurtz treats this as an assault on his person, and acts accordingly.

Klein takes the Doctor back to his cell to wait for his transfer papers to arrive -- or so she claims. In fact, the Doctor knows that she already has his “transfer papers”, or did until he slipped them out of her pocket on the way back. He also has her identity papers, and he knows that they’ve been forged -- and that Klein is therefore desperate to get out of Colditz before the real representative of the German Reich shows up. Before she can stop him, he apparently tears her papers to shreds, stranding her in Nazi Germany with a British accent and no way of proving she is who she says she is. Klein’s horror becomes bemusement when she realises that the “papers” which the Doctor tore up are blank... and he calmly hands the real papers back to her, telling her to get things moving. No doubt the commandant will agree to release him into her custody now that she’s successfully delivered the TARDIS into his hands. This is the case, and Klein recovers her poise as she escorts the Doctor out of Colditz -- past Ace, who is being dragged off to solitary for her attack on Kurtz. He assures Ace that he’ll be back, though Klein may deny this -- for while Klein may think she’s in control, the Doctor is only allowing her this illusion so he can find out who she really is, and where she comes from.

Ace’s predicament has angered the British officers, and Schäfer agrees to discuss the situation with Gower. Officially he can’t listen to accusations against Kurtz, or simply countermand his orders; but unofficially, he does believe Gower’s claim that Kurtz struck the first blow, and he agrees to speak to the commandant. Schäfer is doing his duty, but privately he does sympathise with the prisoners, and he’s been providing Gower with extra rations to share amongst the men; however, while he’s tempted to tell Gower the truth about what he saw inside the TARDIS, he knows this would be going too far. He does keep his word and arrange for Ace’s release, although as punishment for striking a German officer she’ll be placed on domestic duties. As Schäfer lets her out of her cell she demands to know what’s happened to the Doctor, but Schäfer claims that officially, there is no record that either the Doctor or Klein ever existed. Fearing that the Doctor will give the Nazis anything they want as long as she’s a hostage in Colditz, Ace bursts in on Wilkins as soon as she gets the chance, and tells the shocked young man that she’ll help him escape on one condition... they go tonight.

Klein accuses the Doctor of sending his ship away by remote control, but soon realises that he can’t have done so. It seems that they have no choice but to return to Colditz so Klein can take the TARDIS in which the Doctor and Ace first arrived. Meanwhile, the Doctor continues to put the pieces together; history must dictate that he loses his TARDIS to the Germans, and that it remains in their possession until 1965. That will be the year in which Klein finds its flight logs and programmes it to retrace one of its most recent journeys, in order to capture the Doctor and force him to explain properly how his ship works. The Doctor tries to plant the seeds of doubt in her mind, claiming that he would never have lost the TARDIS if she hadn’t travelled in Time and forced him to give up the key. She’s already created a causal loop, and now she risks causing an even worse paradox by taking the TARDIS out of her own past. Or so the Doctor assumes. However, Klein remains confident that she can correct the paradox later -- and if the Doctor refuses to help, then in 1965, the interrogators of the German Reich will force him to do so...

Back in Colditz, Tim sneaks Ace into the sickbay to show her that he’s nearly sawed through the bars on the window. But he hasn’t yet told her that the courtyard beyond is patrolled by a sentry, or that they’ll have to cut through a barbed wire fence to get out. Ace knows that the plan needs a lot of work, particularly when she and Tim are caught -- by Gower, who saw them enter the sickbay without placing a lookout. He’d expected better of Ace, but not of Wilkins; the “escape committee” won’t let Tim in on their plans because they can’t afford to waste an escape route, and Wilkins just doesn’t have the character to make it all the way to the border and across. Tim’s “plan” doesn’t take the second sentry into account -- the one who can’t be seen from the sickbay window, but who carries a machine gun and can see the entire courtyard from where he stands. And they can’t risk attacking either of the guards on their way out; striking a German officer normally carries the death penalty, and the odds of making it to the border and across are just too small to risk it.

Despite these gloomy predictions, Ace insists that she only needs to get out of Colditz for a few days or hours, just long enough for the Doctor to turn the tables on Klein. Gower finally agrees to help her escape, even when Ace insists that they take Tim with them. They will make their break after the final evening roll call; Ace and Tim must pretend to have food poisoning so they’ll be sent to the sickbay, while Gower arranges for a distraction so he can join them. Ace visits Tim in his cell to tell about the plan, but then Kurtz arrives and sends Tim away so he can have some time alone with Ace. In this cell she can scream the walls down if he tries to touch her, but tonight she will be in solitary confinement, and Kurtz can easily dismiss her guard. Ace thus claims to be suffering from food poisoning, and demands to speak to another officer, one who will listen to her. Kurtz allows her a momentary triumph, for he knows she’ll be in Colditz for a very long time -- and sooner or later, he’ll have his chance.

Thanks to Klein’s mention of the German Reich, the Doctor now knows that things are even more complicated than he’d thought; Klein is from a future which never existed, a future brought into existence when the Doctor’s arrival in Colditz changed history. From Klein’s perspective, of course, her history is real and the Doctor’s version is the alternative. As far as she’s concerned, her world is the better one; as a child of German parents growing up in England, she welcomed the Nazi victory. The Doctor points out that she was lucky enough to have blonde hair and blue eyes, but she’s firm in her beliefs. How can he condemn her Reich when he refuses to go back in Time and fight for those enslaved by the British Empire? How can he accuse her of genocide while threatening to wipe out her entire world? In any case, she refuses to tell him anything he can use to change her past; all she will tell him is that he’s too late to save Ace, who will die in a botched escape attempt tonight, when one of her co-conspirators betrays her. The Doctor has heard enough, and before Klein realises what he’s doing, he slips out of his handcuffs, shackles her to a tree, and drops the key just out of her reach. She tries to shoot at him as he flees, but can’t aim properly with her hands cuffed together. The Doctor has one chance to get back to Colditz, save Ace, and restore history to its proper course...

Gower speaks to Schäfer privately, and requests that he dismiss the guard from sickbay tonight. Schäfer refuses to help in what can only be an escape attempt, but although he offers to forget that this conversation took place, Gower won’t accept that... and threatens to turn him in to the commandant for smuggling in extra rations and turning a blind eye to certain activities. Schäfer is enraged by this betrayal at the hands of a man he’d nearly considered a friend, but Gower has never forgotten that they’re on opposite sides. Sadly, as the hour approaches, Tim Wilkins loses his nerve and goes straight to Kurtz. He may tell himself that he’s just trying to save Ace and Gower from getting themselves killed, but Kurtz knows better, especially when the shame-faced Tim requests that he be removed from the list of prominenter in exchange for his co-operation.

As evening falls, so do the Allied bombs. There are more of them with each passing week, and Schäfer has grown weary of it all. When Kurtz presses him about the dismissal of the sickbay guard, Schäfer’s excuses seem tired and rote; unlike the fanatical Kurtz, Schäfer no longer cares which side wins the war, as long as it ends. He’s starting to feel as though he too is a prisoner in Colditz. Kurtz allows Schäfer to go his own way... while he waits near the sickbay for Ace to make her move. Inside, Gower has disguised himself in a German uniform made of boot polish and cardboard; a prisoner once used a similar uniform to convince three guards that he was their superior officer and send them back to barracks. Unfortunately, the fourth guard shot him. For this plan to work, someone will have to knock out the sentry, and although Gower tries to insist that he be the one to do it, Ace points out that she’s already assaulted Kurtz twice; she can’t get into any more trouble, but Gower can. The time has come; Tim must keep an eye on the second sentry, and make his move as soon as it’s clear. They’ll act as soon as the first sentry has passed by one more time...

At the last moment, Ace hits on the one name which can save her -- Klein. Kurtz may be able to convince his superiors that he was forced to shoot Ace because she refused to surrender, but he has been given a direct order to keep her alive as a hostage; and even if Klein believes that he had no choice but to shoot, will she ever trust him again? After a moment’s hesitation, Kurtz angrily allows Ace to live, and takes her back inside to solitary confinement. History as Klein knew it has been changed, as the Doctor learns when he arrives the next morning. Her very presence in her own past is altering events, in ways she can’t have anticipated; she may have erased her future timeline already. Furious, Klein vows to put things right, by ordering Kurtz to shoot Ace at the earliest opportunity...

Gower is placed in solitary, but as Ace is still considered just a girl, she is just confined to her regular cell. However, she’s allowed out for mealtimes... which is where she runs into Tim Wilkins, telling the other prisoners how lucky he was to have bowed out at the last moment. She doesn’t buy a word of it, and publicly accuses him of selling out her and Gower to save his own skin. Schäfer, who isn’t in a very good mood this morning, breaks up the commotion and takes Ace back to her cell... leaving the terrified Tim alone in the middle of a crowd of very angry Allied officers. Meanwhile, the Doctor is locked up next to Gower, and, learning that Gower’s cell overlooks the courtyard, gives him a message to pass on to Ace. Thus, when Kurtz next confronts her, she has information for him -- evidence which he can use to expose Klein as a traitor and get the promotion he desires...

The Doctor is thus in a good mood when Klein next confronts him; Kurtz has double-checked her identity papers, and she’s learned, too late, that the Doctor changed the dates to make them look like the forgeries they are. Fortunately for Klein, Kurtz returned the papers without comment... but the Doctor knows that he’s just being cautious, and that he’ll double-check her credentials with the relevant government department before making his move. Time is against Klein now, and she can’t risk returning to her own era empty-handed, as she took this trip on her own initiative, against orders. The Reich was too frightened of the TARDIS to risk using it, but she felt otherwise -- as did her assistant Schmidt, who helped her to programme the TARDIS for this journey. The Doctor points out that if she takes the TARDIS away from Colditz now she will change her past... but she now reveals that in her version of history, the Doctor escaped from Colditz in any case. The Reich acquired his TARDIS when he inadvisedly returned to Germany ten years later, only to be shot and left for dead by the SS. There will be no paradox if she takes the TARDIS now, as long as she sends it back to 1954 after she’s used it to locate her own, future version of the ship. But as she takes the Doctor into the kommandantur, ready to depart, Kurtz arrives and attempts to arrest her for treason.

The commandant wishes to question Ace and Gower about their escape attempt, and Schäfer thus collects them from their cells. Ace hears Tim wailing for medical attention from his cell, and although at first she’s happy to hear him in pain, Schäfer becomes quite angry when she claims that Tim deserves what he gets for betraying his own side. Subdued by Schäfer’s outburst, Ace finally realises that Tim could easily have been killed by the other prisoners; ironically, he’ll have to be transferred out of Colditz now, for his own safety. Schäfer may be angry with Ace, but he’ll barely even speak to Gower, who realises just how deeply hurt Schäfer was by his betrayal. Events overtake them, however, as shots ring out from the kommandantur and the Doctor runs into them in the corridors, having slipped away as Kurtz and Klein had at each other. Thanks to Klein, he now knows that it wasn’t the TARDIS which changed history after all, and when he questions Ace, she recalls the CD-Walkman which Kurtz confiscated from her earlier. In Klein’s history, Nazi scientists must have studied the lasers in the CD-Walkman, and learned how to use laser technology to refine uranium, thus winning the atomic race. The Doctor, Ace and Gower must remove the CD-Walkman from Colditz before it has the chance to alter history, and although Schäfer tries to stop them from leaving, the Doctor reminds him of what he saw inside the TARDIS, and challenges him to shoot them in the backs as they walk away. Despite himself, Schäfer can’t do so.

The Doctor, Ace and Gower search the kommandantur, find Kurtz’s room and take the CD-Walkman. Ace realises that the Doctor would never have known the truth were it not for Klein -- and this may not have been the stroke of luck it appeared. The Doctor started to suspect the truth when he learned that Klein managed to pilot the TARDIS correctly on her first attempt... with the help of a man named Schmidt. Suppose that history unfolded as Klein claimed; suppose that the Doctor had left Ace’s dead body and the CD-Walkman behind him in Colditz, only to realise that this had changed history. Unable to double back on his own timeline to correct his mistake, he might therefore have returned to Germany, allowed the SS to “kill” him and capture his TARDIS, and then used his next incarnation to infiltrate the Reich and get close to an ambitious scientist... someone who would be willing to risk a journey into her own past, unaware that her “assistant” had programmed the TARDIS to dematerialise after her arrival, leaving her stranded and breaking the causal chain of events which caused her timeline to come into existence. When Ace first arrived, she assumed that she couldn’t be walking into one of the Doctor’s elaborate plans; little did she know...

Now that they have the CD-Walkman, the Doctor and Ace can depart. Ace uses the explosives in her rucksack to create a distraction, and Gower sees them back to the TARDIS... only to find an angry Kurtz waiting for them. Klein has escaped, having driven her stolen car out through the main gate, but Kurtz knew that the Doctor and Ace would make their way here. The Doctor tries to convince Kurtz that he need not fear this technology in the hands of the Allies; that the TARDIS belongs to another world and another time, and humanity isn’t ready for it yet. Kurtz knows that his side needs a technological advantage, and refuses to let them go, but the Doctor wasn’t really talking to him. Schäfer has heard everything, and he steps out of the shadows and orders Kurtz to put his rifle down. Instead, Kurtz accuses Schäfer of treason and shoots him, wounding him in the shoulder.

Gower attacks Kurtz and tries to hold him back while the Doctor and Ace enter the TARDIS, but Kurtz knocks him aside and pursues them, trying to push his way through the doors as they close on him. Trying to stop his enemies from escaping, Kurtz shoots at them, and hits the TARDIS console -- causing the ship to dematerialise while he’s still stuck in the doors, half in and half out of the TARDIS. Gower and Schäfer are shaken by the gruesome results, but at least the Doctor and Miss McShane have escaped; and Gower promises that when he’s questioned about these events, he will testify that Schäfer did his duty. It’s the least he can do, for a man who is as much a prisoner of Colditz as is Gower.

Seeing Kurtz get torn in half has shaken Ace to the core. Not even he deserved a death that horrible. Even if his death wasn’t her fault, she was the one who got Tim Wilkins beaten half to death, and who nearly caused the Nazis to win the war by losing future technology in the past. History may have resumed its proper course now, but Klein is still at large in the past, with knowledge of a future which never happened... and it would be a mistake to underestimate her. Ace needs some time off, and she needs to grow up -- and she’ll start by telling the Doctor to stop calling her Ace. From now on, it’ll be Dorothy McShane.

Source: Cameron Dixon