At Brittanicus Base, Senior Control Technician Jan Garret (Wendy Gifford) and her team are using an ioniser to slow the glaciers rolling over Great Britain. The ioniser helps create carbon-dioxide to enhance global warming. Leader Clent (Peter Barkworth) believes they can prevent an ice-age if they can remain on the glacier-threatened base long enough. Senior Scientist Aren (George Waring) discovers an armoured man frozen in a block of ice on the glacier. Scavengers Storr (Angus Lennie) and Penley (Peter Sallis) bring in the frozen man after an avalanche.
The TARDIS arrives at this point. At first, the time-travellers are to be evacuated like the other Scavengers, but the Doctor helps to repair the failing ioniser, then he examines the thawed “ice warrior” and declares it an alien. While everyone is discussing this, the thawed creature knocks Jamie unconscious and takes Victoria hostage. The creature calls himself Varga, a being from Mars who has been frozen for thousands of years and is now looking for his ship and crew, sealed in the glacier somewhere. He finds four frozen comrades, revives them, and they start to dig out their spacecraft.
The Doctor has ammonium sulphide, which he believes will be noxious to creatures from Mars. Varga is preparing a sonic cannon to attack the base. After much capture and rescue and general running around, Varga invades the base and dismantles the ioniser reactor to get the mercury isotopes he needs for his ship. Pendley raises the temperature in the base to make the Ice Warriors uncomfortable, the Doctor fires off the sonic cannon, forcing Varga to retreat, then uses the ioniser to fire on the ship which is beginning to power up, both destroying the Ice Warriors and halting the glaciation. The TARDIS departs.
Two of the six episodes were lost and replaced with animation. This was the first appearance of the Ice Warriors—reptilian humanoids who have long ago fled a dying Mars—who were admittedly inspired by the Thing from Another World. Their height—the actors were all over six-foot-six—the helmets obscuring their faces, and their serpent-like hissing speech made them quite frightening, at least to Deborah Watling, who was five-feet-tall and, as usual, being dragged off by monsters. The collapsing and cascading glacier made for a literally frightening set.