May 1997 : Benny Summerfield is expecting the Doctor to show up at the house on Allen Road, but she isn't expecting him to be in his eighth incarnation. As she tries to come to terms with the fact that her Doctor is gone forever, a helicopter crashes near the house, and when they arrive they find the only survivor wounded and claiming that "Christian" has escaped. Two policemen arrive, but they seem more interested in reclaiming soil samples from the helicopter than helping the wounded man, and the Doctor realizes that they aren't real policemen. He steals one of the samples and analyses it in the TARDIS, where he discovers that it contains fertile Martian soil. Benny informs him that the first manned mission to Mars since the 1970s is to touch down today, and the Doctor decides to attend the press conference.

Alexander Christian has indeed escaped, and managed to contact his former commanding officer, Alistair Gordon Lethbridge-Stewart. Twenty years ago, while returning from the last Mars Probe mission, he slaughtered his fellow astronauts in a psychotic rage, and has been locked up incommunicado ever since; at least, that's the public story. In fact, the astronauts were killed by native Martians when their expedition stumbled across a civil war, and the British government covered up the truth and held him prisoner with no way of getting word to anyone. He finds the Brigadier more willing to listen to his story than he'd expected. Meanwhile, Home Secretary David Staines orders Veronica Halliwell of MI5 to shut down a publishing company which recently released an expose on UNIT, Who Killed Kennedy, claiming that he has reason to believe a coup is being planned by traitorous elements in the British government.

The Doctor and Benny arrive at the press conference, where Todd, a young man from the Devesham Space Centre slips the Doctor a note asking him to meet later. The astronauts touch down on the surface of Mars, and the press clear the room to report to their bureaus. The Doctor and Benny, however, suspecting that something odd is going on, lag behind and catch a glimpse of a second transmission -- the astronauts have found an artificial gateway. They are expelled from the room before they can learn more, and since they have no way back in the Doctor decides to keep his appointment with Todd. Todd, however, has been murdered, and the Doctor and Benny flee steps ahead of the police, with a set of disks they found in Todd's apartment. They check the disks' contents at the local Internet Cafe, where they discover that the planned touchdown site was changed at the last minute -- and Benny realizes that, whereas the original landing site was in an unpopulated area, the new site is covered with the tombs of Martian warlords.

Lord Greyhaven, the Science Minister whose support made the Mars 97 mission possible, is interviewed by American newscaster Eve Waugh. Impressed by his sophistication and charm, she agrees to meet with him later. Greyhaven then returns to the Space Museum, where he arrives just as the astronauts -- who have stumbled across the tombs of the legendary Ice Warriors -- are found and killed for their sacrilege. The airlocks of the Mars Orbiter then cycle open for no apparent reason, blowing the remaining astronauts out into space to their deaths. The Space Museum imposes a media blackout, and Greyhaven returns to his apartments, where he and Eve make love. Meanwhile, Eve's cameraman, Alan, is contacted by a British conspiracy buff named Oswald, who points out a discrepancy on the recording of the Mars landing; one of the astronauts' oxygen lines had come free from his backpack, and apparently he hadn't noticed. Oswald insists that this proves the Martian atmosphere is safe to breathe, but Alan concludes that the landings were faked.

The Doctor and Benny contact the Brigadier, and take Christian to UNIT HQ to warn Bambera to prepare for an invasion. The astronauts have trespassed in the tomb of an Ice Lord, and the Lord's clan will now be on their way to exact penalties from the clans of the United Kingdom. At that moment, a Martian ship arrives over London, and the Doctor realizes that they arrived far too quickly; they must have left Mars before the astronauts had even arrived in orbit. In accordance with Martian law, the entire United Kingdom is now under the rule of the Argyre clan and its leader, Lord Xznaal. The Prime Minister has just been shot by his own bodyguard while in Washington to deliver a speech, and Staines must enter the Martian ship with Britain's answer to the ultimatum. Lord Greyhaven accompanies him, and Staines, who had believed that the British astronauts were to scavenge alien technology from a dead world and use it to support his coup, realizes that Greyhaven intended to contact living Martians all along.

Greyhaven and Xznaal have been communicating secretly ever since Christian's expedition discovered the truth about the Martians. According to Martian law, Christian should have been returned to Mars for punishment, but since this was impossible he was held incommunicado for twenty years and further missions to Mars were abandoned. American probes to Mars were intercepted and their readings faked to make it appear that Mars was uninhabitable; in fact, the planet is dying, but not yet completely sterile. Now, with the help of Xznaal, Greyhaven has gained control of Britain, and he intends to export Martian technology to the world and make Britain great again. In exchange, human scientists have analysed the dead Martian soil, and found a way to refertilise it. Staines realizes that Greyhaven sabotaged the Mars Orbiter and killed the other astronauts, but Greyhaven refuses to explain why. He appears to have succeeded; only Christian, who was removed from prison to provide expert advice should the Mars 97 mission run into trouble, represents a threat to Greyhaven and Xznaal's plans.

The Doctor uses his sonic screwdriver to attract the Martians' attention, and he and Benny are taken aboard the ship. There, he offers to negotiate a trading agreement which takes all of Earth and Mars into account, rather than just the U.K. and the Argyre clan, but Xznaal has a deeper agenda and tries to kill the Doctor to stop him from interfering. The Doctor and Benny escape when UNIT troops begin shelling the Martian ship, and as the Martians destroy UNIT HQ with their sonic cannon, the Doctor, Benny and the Brigadier retreat to Allen Road. The Doctor contacts Eve Waugh, whom he met at the press conference, and tries to convince her of the truth so she can warn the rest of the world. Eve doesn't believe that Greyhaven is the traitor and killer the Doctor describes, and she thus informs Greyhaven of the Doctor's whereabouts and goes to Allen Road to keep the Doctor occupied. Xznaal is crowned King of England, and then takes Greyhaven and Staines to Kent to witness the attack on the Doctor's house; but Greyhaven realizes, too late, that Xznaal has not told his warriors to spare Eve's life.

The Doctor tries to convince Eve that although the Martian warship has cowed London into submission, the rest of the country has not accepted their rule, and England could soon be plunged into civil war. The Brigadier realizes that Eve is trying to string them along to buy time, and the Ice Warriors then attack the house. Benny manages to dispose of one Ice Warrior using a bottle of vodka and a lit match, and the Doctor uses his sonic screwdriver to counteract the other Warrior's sonic gun; it fires at him anyway, and shatters its own carapace. In response, Xznaal releases the Red Death, a deadly, intelligent gas cloud programmed to isolate the Doctor and kill him. The rich atmosphere of Earth sends the Red Death into a frenzy, however, and it attacks the town of Adisham, killing everyone it encounters. The Doctor sends Benny and the Brigadier to safety while he tries to lure the Red Death away, but when he pauses to save a cat the Red Death falls upon him.

Over the next week, Benny must come to terms with the Doctor's death while helping the Brigadier and Bambera to prepare for their counter-attack. The Martian warship destroys any signs of resistance, however, and the Brigadier realizes that the situation is rapidly becoming status quo. The immediate danger becomes apparent when a man named Raymond Heath contacts them, having escaped from a refinery near Birmingham; although they had originally been working on ways to restore life to the Martian soil, for the last week they have been manufacturing the Red Death, and Heath fled after realizing that the Martians were testing their gas on condemned criminals. The Brigadier realizes that his troops have no choice but to attack the Martians before they can use their gas. The lack of resources on Mars means that the entire invasion force consists of the warship above London; if they can destroy that ship, the invasion will be over.

Alexander Christian arrives in Greyhaven's office and forces him to watch a video containing evidence that the Martians have been conducting experiments upon British prisoners. Greyhaven realizes that Xznaal has violated their agreement and that he will be condemned by history for aiding a monster. He returns to the Space Museum to implement his "insurance policy", but Xznaal realizes that he is missing. Xznaal arrives too late to stop Greyhaven, who has triggered the Mars Orbiter's rockets by remote control; although Xznaal kills Greyhaven, he can then do nothing but watch helplessly as the Orbiter crashes on Mars and obliterates the entire Argyre clan.

Benny accompanies UNIT forces to the Birmingham refinery to destroy the Red Death, but while the soldiers are planting bombs in the storage tanks, she discovers that the gas has already been loaded aboard the Martian shuttle for transport to London. She smuggles herself aboard the shuttle and sends word to Martian High Command that Xznaal is planning to use the resources of Earth to conquer Mars, but before anything can be done, the explosion of the Orbiter renders moot any chance of Martian assistance. Benny is captured by Xznaal's science officer, Vrgnur, and taken back to London, where riots are breaking out as the Resistance moves in and the public rises up against the Provisional Government. The Brigadier is contacted by Oswald, who has been disseminating information to the world via the Internet Cafe, and who has learned that the Martians have transported the Red Death to London. The ship must be destroyed at once.

Benny realizes that Xznaal intends to kill the entire population of Earth to avenge the destruction of his clan. Before he can execute her, however, a giant hologram of the Doctor materializes above London, challenging Xznaal to a final confrontation. While Xznaal is distracted, Christian and Eve free Benny and take her to safety, explaining that the Doctor survived the Red Death by shutting down his body completely; he has spent the past week recuperating and rescuing prisoners from the Martian death camps. Xznaal returns to the warship to confront the Doctor, who has provided Vrgnur with proof that the Red Death will sterilise the entire Earth. Xznaal no longer cares, however, and when Vrgnur tries to stop him from releasing the gas, Xznaal kills him. The Doctor uses his holocamera to broadcast the entire confrontation to the people of London, and the Brigadier has no choice but to order an immediate air strike on the Martian ship.

Xznaal flings the Doctor out of the cargo bay, ten kilometres above London, just as the RAF bombers strafe the ship and destroy it. The Doctor manages to improvise a parachute out of random materials he's picked up and put in his pockets over the past few weeks, and survives his fall. The Provisional Government is disbanded, and the Doctor gives Benny his cat Wolsey and takes her to the 26th century to take up a position as professor of archaeology at St Oscar's University on the planet Dellah. The existence of the Martians is being dismissed as a hoax by people who believe they were invented as a distraction to enable the Provisional Government to keep control of the country. Britain will rebuild and go forward into the future, and for all of the Doctor's friends, the new adventures will continue.