Slithergee patrols throughout the Puxatornee colony are searching for two dissidents called the Doctor and Mel, who were last seen heading for ghetto Delta-Delta. Patrol 14 and his human sight guide, Potter, are searching the streets around said ghetto when a blue box suddenly materialises out of thin air before Potter’s eyes. The blind Slithergee hears the sound and scents something strange in the air, and orders Potter to describe the scene to him.

It is 24 December, 3090, and the Doctor and Mel have come to Puxatornee in search of leptonite crystals with which to defeat the Quarks menacing the space yacht Pinto. However, this may not be as simple as it seems; the TARDIS experiences severe turbulence while materialising, and, according to the scanner, the happy and prosperous colony which the Doctor last visited in 3012 is now a grim urban landscape of slums and workhouses. Nevertheless, the TARDIS sensors are picking up traces of leptonite nearby, and the Doctor and Mel thus emerge to explore.

Potter recognises the Doctor and Mel at once, but his Slithergee master notes a strange odour about the man, unlike any scent the Slithergee has smelled before. They approach cautiously, Potter’s stun pistol at the ready.

As the Doctor and Mel make their way down the street, midnight strikes, marking Christmas Day. They then hear a public address by the colony’s President wishing her people a merry Christmas, extending the hand of friendship to the Slithergees, and announcing that, due to the recent hate crime in ghetto Omega-Beta, the curfew for all humans has been extended to midday, on pain of death. As the Doctor and Mel ponder what they’ve heard, they are confronted and arrested by Slithergee 14 and Potter, who identify the Doctor and Mel by name and accuse them of consorting with known terrorists. Potter keeps the Doctor and Mel covered with his stun pistol and takes them back to back to the central bureau for interrogation, but from the way he defers to the Slithergee it’s clear who’s really in charge. What isn’t clear is how the Doctor and Mel can already be known to these people. Perhaps they’re experiencing the consequences of actions their future selves will make in the past, or perhaps it’s even more complicated than that...

The Doctor and Mel don’t yet know it, but someone else has witnessed their arrival -- a man named Stewart, who tells his associate, a woman named Reed, that Mel told them the truth when she claimed to have a time machine. Rescuing the Doctor and Mel has just become Stewart and Reed’s top priority.

As President Bailey prepares to meet the Slithergee Community Leader, Deputy Mitchell reports that two dissidents have been killed while resisting arrest near ghetto Delta-Delta. Bailey contemptuously dismisses her deputy, who withdraws to his own office when the Community Leader arrives. The Leader insists that Bailey do something about the recent attack on their sight guide training camp or suffer the consequences of her obvious racism against the poor, helpless Slithergees. Reparations must be made in the form of extra protection, more sight guides, and longer curfews for the human population, and in addition, the offensive human superstition of Christmas must be replaced by Slimetide Solstice. When Bailey protests, the Leader accuses her of persecuting his people for their religious beliefs. Dissatisfied, he leaves the meeting and informs Mitchell that the time has come for him to replace the intransigent Bailey. Fearing for his beloved President’s life, Mitchell promises to make her see sense, as he has done so many times before. However, he is interrupted by the sound of alarms as two intruders break into the central bureau -- and he’s stunned when he learns who the intruders are.

The Doctor and Mel are brought before the jolly Professor Capra, who’s having a merry Christmas indeed now that he has prisoners to interrogate. He has spent 15 years perfecting his machine, the mind peeler; the last time he had it up to level 10, the man he was interrogating could no longer hear the questions. Capra starts the mind peeler off on level 1 and gradually increases the power, gently demanding that the Doctor and Mel give him answers that he can believe. Just as he’s about to hit level 5 and start doing permanent damage, Stewart and Reed burst in, shoot him and switch off the mind peeler. As the Doctor and Mel recover, they notice that the mind peeler is powered by leptonite crystals, and the Doctor scoops some up before fleeing with Stewart and Reed -- who already seem to know him and Mel.

Mitchell is forced to inform Bailey that the prisoners have been rescued by terrorists Stewart and Reed, who had, obviously erroneously, been reported killed earlier by a Slithergee patrol. However, Bailey informs him that she had security plant a listening device in his office -- and she now knows that he’s been manipulating her for years in order to keep the Slithergees happy. He’s been doing it because he loves her and fears what the Slithergees will do to her if angered, but she’s been treating him like dirt for years and he now knows she will never love him in return. The man she really loved died 30 years ago, and Bailey now believes that the people who killed him were acting on Mitchell’s behalf, telling her lies about the future so she would surrender her planet to his Slithergee friends. Despairing and furious, she pulls a gun on Mitchell. When the Slithergee Community Leader investigates the subsequent sound of gunfire, Mitchell claims that Bailey shot herself. Bailey is dead and Mitchell is now the President of Puxatornee, and when he receives a report that Slithergee patrol 14 has located the escaped fugitives in ghetto Epsilon-Alpha, he orders the patrol to kill them all.

In the ruins of a toy shop which once belonged to Reed’s grandfather, Stewart and Reed explain to the Doctor and Mel that, 30 years ago, the alien Slithergees arrived in a heavily armed battle cruiser, claiming that their homeworld had been destroyed and requesting living space on one of Puxatornee’s moons. President Bailey gave in to their demands, starting a slow process of appeasement as the Slithergees began to demand more and more. Anyone who spoke out against the Slithergees was accused of a hate crime and executed, including Stewart’s father. Through such reverse discrimination, the Slithergees have slowly gained more and more power, and now humans are second-class citizens and effectively slaves, fit only to serve as sight guides for the blind aliens. But now Stewart and Reed have a chance to change all of that, for somehow they know that the Doctor and Mel have a time machine -- apparently, Mel herself told them -- and they intend to travel back in time and assassinate President Bailey before she can betray her planet to the Slithergees.

The Doctor flatly refuses to meddle with history, but as he argues with Stewart, the toy shop is surrounded by Slithergees led by patrol 14. As the Slithergees and their sight guides warm up their flamethrowers, however, Mitchell calls off the attack, claiming that the fugitives have been located and positively identified in Slithergee district Lambda-Rho. Bewildered, the patrol nevertheless accepts this information and departs, giving the fugitives the opportunity they need to escape. However, Stewart and Reed force the Doctor to take them to the TARDIS, and threaten to kill Mel unless he takes them back in time. The Doctor has no choice but to let them into the TARDIS and dematerialise...

Following a productive meeting with the Slithergee representative committee, the Community Leader informs Mitchell that, despite his past co-operation and efforts on their behalf, the Slithergees have decided that it is no longer appropriate for a human to hold the position of President. As Mitchell protests, the Leader draws a bead on the sound of his voice and guns him down. Now Puxatornee’s future belongs to the Slithergees.

The TARDIS takes the Doctor, Mel, Stewart and Reed back to Christmas Eve, 3060. According to Stewart and Reed, on this night in history, someone broke into the central bureau, tripped a security alarm and killed Bailey’s secretary. Presumably it was an assassination attempt that failed; this time, Stewart and Reed intend to ensure that it succeeds. The Doctor warns them that it’s impossible to predict the consequences of changing the past, but they ignore him and depart, threatening to kill him if he tries to stop them. The Doctor and Mel follow them out of the TARDIS, but soon get lost in the building’s corridors. In order to confuse matters and bring the guards running, the Doctor trips a security alarm, triggering the alert which Stewart and Reed told him about -- but he and Mel then catch sight of Stewart and Reed, and set off in pursuit.

The Doctor is furious, but he has little choice but to take Stewart and Reed back to the TARDIS before they cause even more damage to history. Stewart plants his gun on Clarence’s body, and when the guards arrive, led by officer Potter, they conclude that Clarence shot Bailey and then himself. Potter contacts Mitchell, who is stunned to learn that his beloved Bailey has been found dead in a state of undress with her secretary. Unable to bear the implication that Bailey might have been cheating on him, Mitchell concludes that Clarence must have assassinated her on behalf of the Slithergees -- and, shaken and grieving and not thinking clearly, Mitchell thus declares war on the Slithergees.

The TARDIS takes the Doctor, Mel, Stewart and Reed back to Christmas Day in the year 3090, more or less when they left -- but to a new version of history in which Bailey was killed. Stewart insists that the ends justified his violent means, but the Doctor disagrees. The TARDIS has materialised in what was once Slithergee district Lambda-Rho, but the city is in ruins and the background radiation is uncomfortably high. Stewart and Reed leave, eagerly anticipating their first sight of the bright new future they’ve created, and the Doctor and Mel prepare to put this sorry mess behind them and return to the Pinto. Unfortunately, Stewart and Reed have taken the leptonite crystals, and the Doctor and Mel are thus forced to don anti-radiation gloves and set off after them.

The Stewart and Reed who grew up in this version of history are lieutenants in the colony security forces, and they’re being berated by President Mitchell for allowing two dangerous enemy agents to escape for the second time in as many hours. Oddly, the officer in charge claimed that he’d released the prisoners into the lieutenants’ custody -- but he also claimed, obviously erroneously, that the lieutenants had been shot and vaporised in the ensuing fracas. Since the two enemy agents claimed to know about Professor Capra’s machine, Mitchell sends the lieutenants to guard the Professor’s laboratory and issues a public security alert, telling all guards to be on the lookout for the Doctor and Mel...

As the Doctor and Mel search for Stewart and Reed, they hear the security alert identifying them as enemy agents, and are then caught by officer Potter -- the same man they saw as a Slithergee sight guide in the other history. Potter reports their capture -- or rather, their recapture -- back to Mitchell, who contacts Lieutenants Stewart and Reed and orders them to collect their prisoners from zone Lambda-Rho. While waiting, Mel and the Doctor learn from Potter that Christmas has been abolished in memory of President Bailey, who is now regarded as the first martyr in the war with the Slithergees -- a war which the humans won, but which left Puxatornee a blasted, radioactive wasteland unable to support life.

The Stewart and Reed from the Slithergee history then arrive, stunned by what they’ve seen while exploring this version of history. Assuming them to be the lieutenants, Potter hands over the Doctor and Mel, and, realising Potter’s mistake, Stewart and Reed reject his offer to help escort their prisoners back to the central bureau and send him back on patrol. Once he’s gone, however, he reports to Mitchell, who is surprised to learn that the lieutenants somehow crossed the entire city in such a short time. When he calls Lieutenant Stewart, however, the lieutenant claims that he and Reed are still on the way to collect their prisoners. Suspecting that Potter is in league with the enemy agents, Mitchell orders the lieutenants to return to Capra’s laboratory; he then contacts Capra himself, who seems somewhat distracted as Mitchell warns him to lock the door and keep an eye out for the Doctor and Mel.

Meanwhile, Stewart and Reed demand that the Doctor take them back to the TARDIS. At least when the Slithergees were in charge there was always the chance that things could change -- but in this version of history, the colony will be dead within a generation. Stewart thus orders the Doctor to take him and Reed back to the previous day so they can prevent themselves from going back in time. The Doctor and Mel try to point out that this is impossible, as the “yesterday” they came from no longer exists, but Stewart, beyond reason, threatens to destroy the Doctor’s leptonite crystals unless he agrees to take them back through time again.

The Doctor is thus forced to take Stewart and Reed back to the same place where the TARDIS first materialised, right down to the atom. It’s 10:30 on Christmas Eve, but this isn’t the familiar ghetto Delta-Delta; as the Doctor tried to warn them, Stewart and Reed are still in the new history which they created by killing Bailey. Furious, Stewart returns the Doctor’s leptonite crystals and then sets off with Reed to explore the city, hoping that the Doctor was lying and that they’re in the same colony they remember. Before the Doctor and Mel can enter the TARDIS, however, they’re confronted by officer Potter, the same man who “recaptured” them when they arrived in this history -- but from Potter’s perspective, this is the first time he’s ever seen them. The Doctor therefore claims that he and Mel are Slithergee agents, and the bewildered Potter arrests them and takes them back to the central bureau.

At the central bureau, President Mitchell is complimenting Lieutenants Stewart and Reed on their handling of the food riots in zone Epsilon-Alpha. The lieutenants take no pride in what they did, but food is limited and the fewer mouths they have to feed, the better. Mitchell himself claims to feel no remorse; after Bailey’s death, nothing he’s seen or done can hurt him. Potter then reports in, claiming to have captured two Slithergee agents, and the shocked Mitchell orders Lieutenants Stewart and Reed to interrogate them.

Outside, the other history’s Stewart and Reed find their way to zone Epsilon-Alpha, but where Reed’s grandfather’s toy shop once stood they see hundreds of their fellow human beings lying dead in the streets, victims of a government-sanctioned massacre. They are horrified, especially when they find a statue commemorating the treacherous Bailey as the greatest heroine in history. Stewart finally accepts that their actions have altered history for the worse; he and Reed must rescue the Doctor and Mel from the central bureau and force them to help change history back to the way it was.

Potter locks up the Doctor and Mel in the central bureau -- just as the Doctor had planned, for when they met Potter in the future, he believed that they were enemy agents and claimed that they’d already escaped. Before the Doctor can break out, Lieutenants Stewart and Reed arrive. Mel greets them by name and is surprised when they don’t recognise her, but the Doctor realises that these are different versions of Stewart and Reed who grew up in this version of history. When the lieutenants claim that they’re to be interrogated, Mel wearily prepares to be taken to Professor Capra -- and the lieutenants are stunned when Mel casually reveals that she knows Capra’s machine runs on leptonite crystals and has been under development for 15 years.

The lieutenants leave the Doctor and Mel in their cell and report to Mitchell, who is speaking with Professor Capra himself. Capra is nearly finished, but he admits that he’s still having a problem with the machine’s power supply. Before Capra can elaborate, the lieutenants report to Mitchell that the prisoners know all about the Professor’s work, and Mitchell concludes that the newcomers must therefore be Slithergee saboteurs.

Alone once again, the Doctor resumes trying to decrypt the cell’s cipher lock, and explains to the bewildered Mel that everybody from the other history has a counterpart in this one -- like Stewart and Reed, or like Potter, who was a human sight guide in the other history and a guard in this one. Suddenly, the obvious occurs to him; he and Mel must also have counterparts in this version of history. At ten minutes to midnight, those counterparts will arrive in search of leptonite crystals -- and the counterpart TARDIS will try to materialise in the very same position where the Doctor and Mel’s TARDIS is now standing, with catastrophic results. The Doctor and Mel have less than twenty minutes to pick the lock, get back to the TARDIS and get clear. Fortunately, the Doctor succeeds in picking the lock, and when Lieutenants Stewart and Reed return, they find the cell empty. They report the prisoners’ escape to Mitchell, and he issues a security alert and sends the lieutenants out to search for the Doctor and Mel, who were last seen heading towards zone Delta-Delta.