Plot
 | This article's plot summary may be too long or excessively detailed.(July 2011) |
In the distant future at Brittanicus Base, senior control technician Jan Garrett and her staff struggle to control an ioniser they are using to slow the progress of glaciers rolling over Great Britain. Leader Clent is convinced they can avert a new Ice Age, but the group knows they are only a few hours away from being forced to abandon the base. Tensions rise when Penley, a maverick scientist who has defected from the team, is mentioned. The remaining senior scientist, Arden, is on the glacier searching for archaeological finds, where he discovers an armoured man within a block of ice. Arden and his colleagues dig the ice man from the glacier. Two scavengers observe their actions: the anti-technology Storr and Penley, who live in the tundra. When one of Arden's team is killed in an avalanche, the other two return to base with the ice man. Storr too is injured in the avalanche.
The TARDIS arrives outside the base. The Second Doctor, Jamie and Victoria go inside, where the Doctor helps with the Ioniser. Arden and Walters reach the base with their discovery, and Arden sets up a device to melt the ice around the man. The Doctor examines the frozen man, and they determine that the "ice warrior" is an alien being. An emergency meeting distracts the staff; no one notices that the ice block has melted, with the creature showing signs of life. The creature revives, knocks Jamie unconscious, and takes Victoria hostage.
The ioniser planning meeting is interrupted when Jamie reports that the creature has taken Victoria. However, only Arden and Jamie can be spared for a search party. The creature identifies itself to Victoria as Varga, an Ice Warrior from the planet Mars, who has been frozen for millennia. He insists that Victoria help him find his ship and crew.
Penley goes to the base to steal medical supplies for Storr. He sees Varga and Victoria and follows them as they leave. They encounter Clent, and Varga injures him badly. Penley tries to revive Clent and is found by the Doctor, who has worked out he is the errant scientist. While the Doctor aids Clent, Penley leaves the base.
In the glacier Varga finds four frozen comrades, revives them, and assigns them to create defenses and dig their craft out of the ice. Varga is observed by Penley, who is tracking in the snow having used the medicine on his friend Storr. When Penley returns to Storr, he is surprised to find a visitor, Miss Garrett, who implores him to rejoin the crew of the base.
Jamie and Arden are sent into the glacier, ostensibly to find the alien spacecraft rather than Victoria. They discover the Ice Warriors' cave excavation and report this to base. They are ambushed by the Ice Warriors and left for dead. Penley finds Arden dead, but Jamie alive. Penley takes him back to his home. Storr decides to speak to the Ice Warriors, convinced they might be allies.
Having failed to contact Arden, the base personnel assume something bad has happened. The video link appears, operated by Victoria, who tells them of the danger of the Ice Warriors. An Ice Warrior, Turoc is sent to capture Victoria again and use her as bait. The Doctor decides to go to the spaceship and rescue her. Before leaving, he takes a phial of ammonium sulphide, which he deduces will be noxious to the aliens. Victoria flees into the icy caves. When the Ice Warrior finds her, he is caught in an avalanche and crushed.
An examination of the engines of the Martian craft reveals them to be functional but low on fuel. When the Ice Warriors encounter Storr, they reject his offers of help. Storr is killed but Victoria, whom he brought from the ice caves, is permitted to live.
Penley finds the Doctor and takes him to Jamie. He determines that Jamie has temporary paralysis and heads to the Martian craft. He offers himself as an envoy, leaving his communicator active so Clent can hear, and is allowed to enter the airlock. With the glacier threatening to crush the spacecraft, the Doctor has Victoria released to him. Before Varga takes the communicator, the Doctor relays the message that Clent needs to use the Ioniser, regardless of consequences. The Doctor is marched to the core of the spacecraft, where he spots an ion propulsion system. Varga decides to attack the base and orders his Warriors to prepare a sonic cannon.
Penley brings Jamie to base on a motorised sled. Clent gives Penley a frosty reception, and they bicker. Clent says he has decided to use the Ioniser. Zondal has been given the task of arming the sonic cannon. The Doctor and Victoria release the chemical solution at Zondal, who collapses, but his hand activates the sonic cannon as he falls.
The sonic blast triggered by Zondal glances the base, causing minor damage. Varga calls Clent, threatening to fire again unless the humans surrender. Clent knows the base dome cannot survive another sonic blast and suggests a peace meeting. The talks fail when a demented technician, Walters, tries to shoot the Martians. Varga dismantles the Ioniser reactor to get the mercury isotopes he needs for his ship. Without the ioniser, the glaciers move forward.
The Doctor and Victoria adjust the sonic cannon so it will only harm the Ice Warriors. Penley alters the temperature and atmosphere controls in the base so it becomes uncomfortable for the Martians. The Doctor fires the sonic cannon, forcing Varga and his men to retreat. He fuses the sonic cannon before he and Victoria flee the ship. The Doctor and Penley recalibrate the Ioniser. The computer calculates a fifty-percent chance that the Ioniser will explode when trained on a spacecraft with an ion engine; Penley tells Clent to work without the advice of the computer. When the computer overloads, Penley takes charge and starts the Ioniser.
The Martian craft begins to power up but is destroyed by the Ioniser. The ship explodes without starting a chain reaction, which solves the problem of the Ice Warriors and the glacier. The Doctor, Jamie and Victoria depart as green shoots emerge through the melting snow.
