Romana warns Arkadian that Charley Pollard is a personal friend, and refuses to discuss the rumours that she was once a conduit for anti-Time. However, Arkadian assures Romana that his interest in Sissy Pollard is entirely unrelated to these events; this is old business left unresolved from some time ago. All he asks is that Romana turn a blind eye to his activities for the next three hours, and keep Co-ordinator Narvin off his back; in exchange, Arkadian will give Romana the name of the person responsible for the theft of the Time Lords’ timonic fusion device and the subsquent attempt to spark a war between the temporal powers on Gryben. Romana reluctantly agrees to his terms when he assures her that his work will do nothing to affect established history, and he leaves, sticking her with the bill for dinner.

Elsewhere on the train, Sissy Pollard changes compartments when a lower-class family enters her own. Her new travelling companions are the elderly Miss Muriel Joy and Leela, whom Sissy assumes is a Gypsy queen. When Romana arrives and identifies Sissy as Charley’s sister, Sissy is quite put out and demands to know how these strange women know her missing sister. Someone then flags down the train and boards it, and though Sissy fears that her “boyfriend” Erich Kepler has tracked her down, it is in fact Narvin. As the train starts moving again, Romana goes to head off Narvin as per her arrangement with Arkadian, leaving Leela to guard Sissy. Narvin is approaching the compartment, tracking a source of temporal displacement, but as he and Romana argue over jurisdiction, the tracker suddenly goes wild -- and, through the window, they see another, identical train rapidly approaching their own on the same track.

The conductor arrives at the women’s compartment, and Sissy realises that she neglected to purchase a ticket in her rush to leave. Since Leela has no ticket either, she overpowers and ties up the conductor when he enters, only to recognise him as Torvald. Leela seizes the opportunity to avenge her husband’s death, intending to tell Romana that she killed Torvald by accident. But before she can do so, she senses something wrong and sees the second train speeding up behind them. Leela grabs Sissy to protect her moments before the trains collide. To their surprise, they find themselves not only alive, but alone in the compartment. Sissy assumes that they passed out, but Leela does not believe this to be the case -- and when she tries the door, she finds it locked from the outside.

Romana and Narvin arrive at the compartment, only to find the bewildered Torvald and Miss Joy alone. Narvin orders Torvald to erase Miss Joy’s memory once the affair is complete, and examines his temporal tracker only to find that Sissy -- and the other train -- are apparently right in front of him. Arkadian then arrives, much to Narvin’s irritation, and explains that he’s finishing up old business from the last time he visited this era. For reasons which he won’t go into yet, he and an associate who goes by the name of Erich Kepler intervened in Sissy Pollard’s suicide attempt. However, Erich turned out to have plans of his own, and instead of turning Sissy over to Arkadian, he made off with the girl himself. By the time Arkadian tracked him down, Sissy had vanished from the train. Arkadian then became involved in other dealings, but now he has returned to sort matters out by sending Sissy a fake telegram which got her out of Munich before Erich could get to her, thus creating a slightly different version of history. However, it seems that Erich has used advanced temporal technology to merge the two timelines -- and since Arkadian’s telegram was such a minor change to history, Sissy ended up at exactly the same point in time and space. When the trains converged, the primary timeline reasserted itself, and Sissy was dragged across -- along with Leela, who was holding onto her at the moment. This timeline still exists, however, because Leela’s presence could still significantly change history -- and since Romana brought her here, this means Romana is responsible.

In the primary timeline, Leela learns that Sissy is a member of the League of English Fascists; she has met personally with Hitler, whom she considers to be a charming and insightful man, and was appointed as the League’s Munich representative by Sir Oswald Mosely himself. Leela has personal experience with the madness that results when people are separated into different groups, and when Sissy insists that people like Leela really can’t be allowed to drag the better people down to their level, Leela points out that she’s the one who has just picked the lock of the compartment, and invites Sissy to step out and face the danger herself. Sissy hesitates, and Leela, her point made, emerges to investigate. The other passengers appear to have been knocked out with sleeping gas, apart from Leela and Sissy, who weren’t in this timeline when the gas was released on the train.

Leela and Sissy are then confronted by the man who brought them here: Erich Kepler, who scoffs at Sissy’s blatant attempts to curry his favour. He has no interest in obtaining sexual favours from such a one-dimensional, empty-headed Nazi groupie -- for he is a Time Lord. Leela identifies herself and accuses him of treachery, but Erich claims to be a loyal agent of the CIA. As far as Leela is concerned, this just puts him in the same company with snakes and murderers like Narvin and Torvald, but Erich, amused, reveals that he is Torvald himself, in an earlier incarnation than the one Leela knows. However, while he’s distracted, Sissy, quite upset by his beastly behaviour towards her, kicks him somewhere extremely painful with her spiked heel, and she and Leela run for it.

On the other train, Miss Joy, apparently quite bewildered by all that’s going on, excuses herself to powder her nose. Romana orders Torvald to let the innocent old lady go about her business, and Narvin, irritated, orders Torvald to escort Romana back to her TARDIS to ensure that she does no further damage. Before she can go, however, Arkadian advises her to reconsider going anywhere alone with Torvald, and reveals that Erich Kepler was Torvald himself in a past incarnation. Naturally, Torvald denies the accusation, but Narvin then reveals that Arkadian has already supplied him with proof. Narvin had already become suspicious of Torvald’s recent strange behaviour, and thus struck a deal of his own with Arkadian, agreeing to turn a blind eye to Arkadian’s sending a telegram to Sissy in exchange for his help to manipulate Torvald into this trap. Once Erich makes his move against Sissy, Narvin will be free to arrest Torvald for his past self’s crimes.

Torvald insists that he is not guilty of this crime, and, desperate, he pulls out a staser, stuns Narvin and forces Romana back to her TARDIS at gunpoint. There, K9 Mark II warns her that this timeline is now unstable; when the train on the primary timeline reaches its destination, the secondary timeline created by the telegram will cease to exist. Torvald orders Romana to jump to the primary timeline, convinced that Arkadian will have his own way out and that he’ll know better than to abandon Narvin. Romana refuses to let Torvald interfere in his own past, and demands to know what he wanted with Sissy; however, he claims to have no memory of these events, and points out that, if he were to interfere in his own past, Romana would have to wipe his past self’s memory of the event to restore the timeline. Romana thus seems to have little choice but to carry Torvald over to the primary timeline with her.

Aboard the train, Leela realises that she has a chance to kill Torvald in his past incarnation before he kills her husband -- but before she can do so, she and Sissy are unexpectedly attacked by a vicious alien monster. They retreat the way they came, and try to bar the door against the monster with a fire axe; however, Erich catches them and grabs Sissy before she can pass the axe to Leela. Much to their surprise, Miss Joy then knocks on the door, claiming that something is pursuing her -- and while Leela is caught off guard, Miss Joy smashes through the door and transforms back into her other, monstrous form, knocking Leela senseless.

Miss Joy is Arkadian’s client, a shape-shifter who boarded the train to ensure that Sissy was delivered safely -- and, once she learned what had happened, she leapt off the train and then back on, thus switching to the dominant timeline. Miss Joy is a dealer in rare artefacts -- and she intends to sell the stuffed and mounted Sissy to one of the tinpot tyrants out in the frontier worlds as a rare Nazi collectible. Sissy insists that she’s not a Nazi, only an observer, but Erich scoffs; she’s seen the hunting parties at Nuremburg, and her apartment in Munich once belonged to a couple called the Goldbergs, who have never been seen since Sissy moved in. She may not have ordered these things directly, but she turned a blind eye to them, and that makes her just as much a Nazi as the people she’s been “observing”. Arkadian only interfered in her suicide because the bullet would have made the taxidermist’s job more difficult.

Miss Joy prepares to collect her prize, shrugging off Erich’s attempts to shoot her; bullets have no effect on a mesomorph. But when Leela rises up behind her and plants the fire axe in her skull, that does the trick. Leela then pulls the axe out of the mesomorph’s remains and turns on Erich, demanding that he surrender Sissy. Leela despises Sissy and all that she stands for, and she believes that Charley would feel the same way, but she will not pick and choose whose life is worth defending. At that moment, however, Romana’s TARDIS unexpectedly materialises on the tracks ahead of the train -- and Erich flings Sissy out of the train and jumps out himself, just ahead of the furious Leela.

Torvald is shocked when he emerges from the TARDIS to find that Romana has materialised, not aboard the train, but in front of it. Romana is sick of these silly boys treating the timeline like their personal model railway set, and unless Torvald reveals the entire truth, she’s going to let her TARDIS derail the train. Torvald begs her to stop before she kills Leela, but Romana doesn’t believe that he really cares about Leela’s fate -- and, with seconds to go before the collision, Torvald breaks down and confesses that Leela is his wife.

The train does not collide with Romana’s TARDIS, which dematerialises just in time. Leela has survived her leap from the train, as has Sissy, but Erich has broken both of his legs. Narvin and Arkadian arrive in the primary timeline, and as Arkadian searches for Leela, Romana sends Sissy back to Munich to fulfill her role in history. She then demands an explanation from Erich, who reveals that he’s disgusted with Romana’s liberal policies and her dealings with the lesser species and thus decided to do something about it. Sissy’s sister Charley should have died when the R101 crashed, but she was saved by a personal friend of Romana’s -- and because Romana turned a blind eye to his transgression, the Universe was nearly destroyed by the forces of anti-Time. What has happened once could happen again, and Erich thus tracked down one of Charley’s closest family members and altered the course of history by saving her life. Unless Romana confessed to her “crimes”, resigned from office and allowed the Time Lords to erase her Presidency from history, Erich intended to allow Sissy to live, spreading the taint of anti-Time throughout the Universe again. Even Narvin considers this a foolhardy plan, and the disgusted Romana tears up the “confession” Erich had prepared for her.

Arkadian finds and wakes Leela, who rushes to warn Romana of Erich’s true identity. She then attacks Torvald, intending to kill her husband’s murderer as his past self watches; however, Romana stops her and reveals that the man they’ve known as Torvald isn’t Torvald at all. Leela listens, stunned, as “Torvald” admits that it wasn’t Andred who died during the shoot-out on Gallifrey. Just as Narvin claimed, Andred heard rumours of an isolationist plot against Romana, and, suspecting that Narvin was involved, he attempted to infiltrate the CIA and learn more. In fact, Narvin had nothing to do with it, and, suspecting that Andred was plotting against him, he sent Torvald to meet with Andred in his place. When Torvald and Andred met, they exchanged gunfire -- but in truth, it was Torvald who was killed and Andred who regenerated. Andred seized the opportunity to get inside the CIA, identified himself as Torvald to the rescue parties, and switched the real Torvald’s biodata extract with his own. By the time his post-regenerative confusion cleared and it occurred to him what he was doing to Leela, however, it was too late -- and, ever since, she has believed her husband to be dead and Torvald to be her enemy.

Leela is stunned when the truth sinks in, and coldly rejects his attempt to apologise. She had grieved for her husband’s death, and still considers him dead to her. Leela stalks off in the rain as the tear-stricken Andred follows, begging for her forgiveness -- and “Erich”, the only real Torvald here, bursts out laughing. Romana will now erase Torvald’s memories, and he will go on to be shot and killed in a gunfight, while Sissy commits suicide in Munich and Andred betrays his wife and destroys their love. And Romana has no choice but to let it happen in order to preserve the timeline. Furious, Romana orders Narvin to wipe Torvald’s memory, and Narvin is only too happy to do so, especially since the process hurts.

Romana leaves Narvin to his business and tracks down Arkadian, who seems quite amused by the way things have turned out. Since Romana kept her side of the bargain, he keeps his -- and reveals that it was Torvald, the real Torvald, who was responsible for the timonic fusion affair. He was the ringleader of the isolationists on Gallifrey, and, knowing that he could be discovered and killed at any moment, he stole some of the android servitors being prepared for the peace summit and pre-programmed them to set these events in motion after his death. So Arkadian claims, but there’s no way to prove it now, since Narvin has just wiped the past Torvald’s memory, and in Romana’s era, Torvald is dead. Romana points out that an arms dealer such as Arkadian would stand to profit from a war between the temporal powers -- but she has no way to prove her suspicions, and must allow Arkadian to depart, scot-free.