Kirk, Spock, and McCoy transport an ailing Commissioner Nancy Hedford from Epsilon Caneris 3 to the Enterprise. Their shuttle is intercepted by an ionized hydrogen cloud and forced to land on a small inhabitable planet. The shuttlecraft engine is damaged and while they struggle to make repair, they come into contact with a person of great historical meaning to the Federation, thought to have been dead for 150 years.

In the 30th century, Earth is experiencing a new Ice Age, and bases all across the world are struggling to hold back the advancing glaciers. Brittanicus Base is running into difficulty, however, as Leader Clent's stubborn reliance on the base computer has alienated the humanist scientist Penley, who has deserted the base and gone out to live in the snowy wilderness outside. When the Doctor shows up, Clent enlists his help to get the malfunctioning Ioniser back on-line. However, more problems ensue when the frozen body of a Martian warrior is discovered in the nearby glacier. If an alien spacecraft is buried in the glacier, then use of the Ioniser at full power could cause its power source to explode and irradiate all of Britain. And then the Ice Warrior unexpectedly returns to life, kidnaps Victoria, revives the others of its kind and prepares to attack the presumably hostile base...