Security patrols throughout the Puxatornee colony are searching for two enemy agents called the Doctor and Mel, who were last seen heading for zone Delta-Delta. Lieutenants Stewart and Reed are searching the streets around said zone when a blue box suddenly materialises out of thin air before their eyes...

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, dangerously radioactive urban wasteland. Nevertheless, the TARDIS sensors are picking up traces of leptonite nearby, and the Doctor and Mel thus don anti-radiation gloves and emerge to explore.

Lieutenants Stewart and Reed recognise the Doctor and Mel at once, but are confused by their behaviour; the two fugitives aren’t even trying to hide. The lieutenants approach cautiously, their vaporisation guns at the ready.

As the Doctor and Mel make their way down the street, midnight strikes, marking Christmas Day -- or so they think until they hear a public address by the colony’s President wishing his people a sombre Retribution Day and announcing that, due to the recent disturbance in zone Epsilon-Alpha, all citizens are to remain indoors on pain of death. As the Doctor and Mel ponder what they’ve heard, they are confronted and arrested by Lieutenants Stewart and Reed, who identify the Doctor and Mel by name and claim that they’ve already confessed to being Slithergee agents and knowing of “the Professor’s machine.” The lieutenants drive the Doctor and Mel back to the central bureau for interrogation, and on the way, Mel wonders how they 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 sombre Professor Capra visits President Mitchell to tell him that, after 15 years’ work, the experiment is now ready to proceed. He requests permission to visit his family one last time, but Mitchell reveals that, since Capra’s work was of top priority and there wasn’t enough food to go around, the rest of his family was “rationalised.” Capra is horrified, but realises that if he refuses to proceed with the experiment his family will have died in vain. Mitchell then receives a call from Lieutenants Stewart and Reed, reporting that the Doctor and Mel have been recaptured and that they are resuming the interrogation.

The Doctor is unable to decrypt his cell’s cipher lock; according to Lieutenants Stewart and Reed, he’s escaped that way once already. Still unsure what he and Mel are being accused of, the Doctor promises to reveal all if the lieutenants explain what a Slithergee actually is. The lieutenants thus explain 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. Before President Bailey could decide how to respond to their demands, she was assassinated by a Slithergee agent. The colonists retaliated, driving away the Slithergees, but at great cost. Anyone who spoke out against the war was accused of treason and executed, including Stewart’s father. The planet has been reduced to a radioactive wasteland incapable of supporting life, and the colony will be dead within a generation. But now the colonists have a chance to change all of that, for Professor Capra has built a time machine, and Lieutenants Stewart and Reed intend to travel back in time and save President Bailey’s life, giving her a chance to make peace with the Slithergees.

President Mitchell can’t stop thinking about the events of Christmas Eve, 3060. There was a security alert, and when the guards responded, they found that President Bailey’s secretary, Clarence, had apparently shot her and then turned the gun on himself. Oddly, however, the two were in a state of undress at the time -- and that’s what Mitchell just can’t understand. He loved Bailey dearly, and can’t bring himself to believe that she might not have loved him as much in return. He has thus become obsessed with his loss, and is convinced that, had Bailey lived, she would have secured a lasting peace. The presence of two actual Slithergee agents at this vital time thus disturbs him greatly, and he sends officer Potter to check up on the Doctor and Mel and find out what they’re doing on Puxatornee.

Lieutenants Stewart and Reed return to patrol, leaving the Doctor and Mel to ponder what they’ve learned. As the Doctor tries to work out how to escape and prevent the lieutenants from changing the course of history, Stewart and Reed apparently return -- but they’re behaving strangely, and the Doctor soon realises that these people are not in fact the two lieutenants who just interrogated him and Mel. Whoever they are, they know that the Doctor and Mel have a time machine called the TARDIS, and they’ve managed to impersonate the lieutenants and convince the guards to release the Doctor and Mel into their custody. However, as they try to get out of the central bureau, Potter reports to Mitchell that the lieutenants are leaving with the Doctor and Mel. Mitchell calls Lieutenant Stewart to demand an explanation, only to learn that he and Lieutenant Reed are currently on patrol in zone Omega-Beta. Mitchell orders Potter to stop the impostors by any means necessary, and in the ensuing fracas, the “false” Stewart and Reed are shot and vaporised. The Doctor and Mel escape, but as Potter pursues them towards zone Lambda-Rho, they double back, shaking him off. The Doctor begins searching for Capra’s laboratory, intending to sabotage the time machine while the guards are searching the city.

Mitchell issues a public security alert telling all guards to be on the lookout for the Doctor and Mel, and sends Lieutenants Stewart and Reed to guard Capra’s laboratory. When the Doctor and Mel locate the laboratory they find the lieutenants standing guard outside, but Mitchell then contacts the lieutenants and informs them that Potter has just arrested the Doctor and Mel in zone Lambda-Rho. Lieutenants Stewart and Reed set off to collect them, and as soon as the coast is clear, the Doctor and Mel break into the laboratory. Inside, they bluff Capra into believing that they’re armed and dangerous, and when Mitchell contacts Capra, ordering him to lock the door and keep an eye out for the Doctor and Mel, Capra is too frightened to admit that they’re already there.

Meanwhile, Potter is in serious trouble. He claims to have arrested the Doctor and Mel and handed them over to Lieutenants Stewart and Reed, who rejected his offer to help escort their prisoners to the central bureau. But the lieutenants couldn’t have possibly have travelled from Capra’s laboratory to zone Lambda-Rho in that time, and indeed, when Mitchell contacted them they claimed that they were still on their way. Unable to explain this, Potter suggests that the lieutenants may be in league with the Doctor and Mel. It’s the only explanation that makes sense, and if it’s true, it means that Stewart and Reed must intend to sabotage Capra’s experiment -- and Mitchell has just ordered them to return to his laboratory. Mitchell sets off to stop them, but first vaporises Potter for his incompetence.

While trying to sabotage Capra’s time machine, the Doctor finds that it’s powered by leptonite crystals. Before he can cause any serious damage, however, Lieutenants Stewart and Reed burst in and force him to back away from the machine. Capra activates the time machine, but Mitchell then arrives and orders Capra to stop, accusing Stewart and Reed of treason. But once turned on the machine can’t be turned off. In his rage, Mitchell lets slip that he’s always known that Bailey’s secretary wasn’t a Slithergee agent, and Stewart and Reed realise that the war was the result of Mitchell’s misplaced anger and jealousy. But as far as they’re concerned, this makes it all the more vital that they prevent it from happening. Mel rushes forward to intervene, and the unstable time field sweeps her up, transporting her into the past with Lieutenant Stewart. The field then collapses, leaving the others unable to follow or retrieve Mel and Stewart. The Doctor removes the leptonite crystals from the machine, gives them to Lieutenant Reed for safekeeping, and takes her back to the TARDIS, which is now their only means of tracking down Stewart and Mel.

Back in the laboratory, Capra informs Mitchell that he never did solve the power supply problem -- and that the time machine can’t be switched off. Before Mitchell can respond, the machine overloads and explodes, destroying far more than just the city. Now Puxatornee really does have no future...

The TARDIS takes the Doctor and Lieutenant Reed back to Christmas Eve, 3060. According to Reed, on this night in history, someone broke into the central bureau and tripped a security alarm, but Bailey was still assassinated before the guards arrived. This time, Stewart intends to ensure that the assassin fails. The Doctor warns Reed that it’s impossible to predict the consequences of changing the past and demands that she help him find and stop Stewart, but they 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 Reed told him about -- but he and Reed then catch sight of Stewart and Mel, and set off in pursuit.

Lieutenants Stewart and Reed tell Bailey that they are from a future in which war was declared with the Slithergees, and insist that she make peace with them, whatever the cost. The Doctor is furious, but has little choice but to take the lieutenants back to the TARDIS, as the damage to history has been done and they had planned to vaporise themselves after their task was complete. When the guards arrive, led by officer Potter, Bailey claims that she shot Clarence in self-defence. Potter contacts Mitchell, who is stunned to learn that his beloved Bailey was nearly killed by her secretary -- and Bailey, shaken and grieving and not thinking clearly, orders him to accede to the Slithergees’ demands and let them settle on the first moon.

The TARDIS takes the Doctor, Mel and Lieutenants 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 not killed. Lieutenant Stewart insists that the ends justified his violent means, but the Doctor isn’t so sure whether Clarence really was going to kill Bailey as history claimed. The TARDIS has materialised in what was once zone Lambda-Rho, but there is no background radiation -- and the buildings in the city now resemble concrete honeycombs. Lieutenants 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, the Doctor remembers too late that Reed is safeguarding the leptonite crystals, and he and Mel are thus forced to set off after her and Stewart.

Even in this version of history, Bailey is dead and Mitchell is the President of Puxatornee -- but he’s being berated by the Slithergee Community Leader because there are no Slithergees on the human representative committee. Mitchell points out that the Slithergees refuse to allow humans on the Slithergee representative committee, but the Leader claims that no human can understand the ethnic experience of being a Slithergee and accuses Mitchell of discrimination. The Leader sets off to confer with his fellow Slithergees on the matter; meanwhile, Mitchell receives a report that Slithergee patrol 14 has located escaped fugitives in ghetto Epsilon-Alpha, and orders the patrol to kill them all.

As the Doctor and Mel search for Lieutenants Stewart and Reed, they find notices identifying this district of the city as a Slithergee district. It seems that in this history, the humans and the Slithergees live alongside each other, but at what cost? The lieutenants then arrive in a panic and usher the Doctor and Mel into a nearby pipeline to hide from the giant slug following them through the streets. This is a Slithergee, and it’s using a human on a leash like a seeing-eye dog. The lieutenants reveal that they’ve seen a statue of Bailey defaced with angry graffiti which makes it clear that, rather than making peace with the Slithergees, she effectively surrendered the planet to them. They demand that the Doctor take them back to the TARDIS, but the Slithergee and its sight guide are currently in the way...

The Slithergee in question is in fact the Community Leader, and his sight guide has recognised the four fugitives. The Leader reports to Mitchell, who contacts Slithergee patrol 14 and calls off the attack in Epsilon-Alpha, claiming that the fugitives have been located and positively identified in Slithergee district Lambda-Rho. Bewildered, the patrol nevertheless accepts this information and sets off for Lambda-Rho. As soon as patrol 14 arrives with his sight guide, the Leader sets off again to attend his meeting with the committee. Patrol 14 is surprised to find that he can smell the fugitives whom he’d already scented in Epsilon-Alpha; confused, he sends his sight guide, Potter, to search the drainpipes for the fugitives. Potter soon locates the Doctor and Mel -- and is stunned to see them with Stewart and Reed, whom he claims to have killed already. Stewart guns Potter down, vaporising him, and he, Reed, Mel and the Doctor quickly retreat to the TARDIS and dematerialise before the Slithergee notices that his sight guide has gone missing.

Lieutenants Stewart and Reed are stunned by what they’ve seen in this version of history. At least when the planet was a radioactive wasteland, the human colonists still had their dignity -- but in this version of history, they are second-class citizens and effectively slaves, fit only to serve as sight guides for the blind Slithergees. Lieutenant Stewart thus orders the Doctor to take him and Lieutenant 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 Lieutenant Stewart, beyond reason, threatens to kill Mel unless the Doctor agrees to take them back through time again.

The Doctor is thus forced to take Lieutenants 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 zone Delta-Delta; as the Doctor tried to warn them, Lieutenants Stewart and Reed are still in the new history which they created by saving Bailey. He demands that Lieutenant Reed return the leptonite crystals; she does so and then sets off with Lieutenant Stewart 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 forced to hide from Slithergee patrol 14 and Potter, the same man whom they saw killed when they arrived in this history. Stewart and Reed then arrive, demanding to know why the Doctor and Mel are breaking curfew -- and when Mel greets Stewart and Reed by name, they are bewildered, as this is the first time they’ve ever seen her.

At the central bureau, Deputy Mitchell informs President Bailey -- who is still very much alive -- that the Slithergee Community Leader is requesting a meeting to discuss the terrorist attack in ghetto Omega-Beta. Bailey knows that she’ll have to make more concessions as reparations for the attack... but she can’t help but notice that the Slithergees evacuated the district shortly before the explosion, and that only their human sight guides were killed. As he has always done, Mitchell reminds her of the consequences should she offend the Slithergees, but she no longer has any stomach for his “advice” -- and she’s beginning to wonder why the attack apparently came without warning, even though they’d arrested and interrogated several members of a terrorist cell the previous week. She dismisses Mitchell, claiming that she must prepare her Christmas broadcast, but first she contacts her interrogator, Professor Capra, to ask him personally about the interrogation of the terrorists. Capra reveals that the terrorists were subjected to his mind peeler and revealed everything they knew about Stewart and Reed’s plan to destroy the sight guide training centre in Omega-Beta -- and that Capra passed the information on to Deputy Mitchell. Bailey, furious, calls security and orders them to plant a listening device in Deputy Mitchell’s office...

The Doctor and Mel take shelter with Stewart and Reed, but Mel doesn’t understand why they don’t recognise her, and before the Doctor can stop her she reminds them that they’ve all just arrived here by time machine. The Doctor realises that these are different versions of Stewart and Reed who grew up in this version of history, and this version of Stewart is very interested in Mel’s claim to have a time machine. The four are then forced to scatter when a Slithergee patrol approaches, and the Doctor and Mel manage to ditch Stewart and Reed. Mel realises that this history’s Stewart and Reed must be the same pair who rescued her and the Doctor from the prison cell in the other history -- but how did they manage to get there in the first place? The obvious suddenly occurs to the Doctor; like Stewart and Reed, and like Potter -- who was a guard in the other history and a sight guide in this one -- the Doctor 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. Despite the danger of running into Slithergee patrols on the way, the Doctor and Mel have less than twenty minutes to get back to the TARDIS and get clear.

Elsewhere, Lieutenants Stewart and Reed find their way to ghetto Omega-Beta, where they see hundreds of their fellow human beings lying dead in the streets, victims of a terrorist attack. They are horrified, especially when they find a sign indicating that this was a sight guide training camp. Lieutenant Stewart finally accepts that their actions have altered history for the worse; he and Lieutenant Reed must find the Doctor and Mel and force them to help change history back to the way it was. However, before they can get back to ghetto Delta-Delta they are arrested by Slithergee patrol 14 and Potter, who identify them by name as terrorists and demand to know the identity of the two others they were spotted consorting with. Lieutenant Stewart realises that he’s talking about Mel and the Doctor, and the Slithergee takes note of the names and orders Potter to execute Stewart and Reed. Lieutenant Stewart finds that his own gun has run out of power, and Lieutenant Reed remembers -- too late -- that when they first arrived in this history, they met Potter, who claimed to have killed them already. Before she or Stewart can react, Potter shoots them both dead. Slithergee 14 informs Mitchell that Stewart and Reed have been killed resisting arrest, and Mitchell issues a security alert and sends patrol 14 out to search for the Doctor and Mel, who were last seen heading towards ghetto Delta-Delta.