Kirk and a landing party beam down to examine tritantium deposits and are attacked by a gaseous creature that feeds on red blood cells. Kirk remembers the creature as one that he fought - and failed to destroy - earlier in his career and where his mistake killed half the crew. Now the creature is stalking the Enterprise and Kirk is determined to destroy it no matter what the cost.

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...