Kirk and a landing party investigate a colony on Gamma Hydra 4 and find that most of the residents have died... of old age. They bring the last few survivors on board the Enterprise where they die, but it soon becomes apparent that everyone from the landing party is showing the same signs of accelerated aging. Only Chekov appears immune and they must find a cure before they all die of old age.

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...