Another adventure for everyone's favourite Time Lord. The Doctor (Tom Baker), Sarah and Harry land on space station Nerva, where the chosen few of the human race are in suspended animation, awaiting the call to revive. However, an insectoid alien life form known as the Wirrn has infiltrated the Ark and cut through the humans' alarm clock. The Wirrn now intend to use the sleepers as the hosts for their hatchings and claim the Earth as their own.

The newly regenerated Doctor (Tom Baker), assisted by Sarah, Harry and the Brigadier, has to track down the theft of the components for a powerful new laser gun, and the death of a prominent politician. Sarah investigates the mysterious Think Tank, where it becomes clear that robot K-1 is being misused by its political extremist masters and the Doctor has to prevent the robot from being used to start an atomic war. Tom Baker's first story as the Doctor.