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.

Jon Pertwee stars as the time travelling Doctor, who once more finds the planet under threat. Trying to avert a war which will bring Earth under the domination of the Daleks and their ferocious slaves (the Ogrons), the Doctor fights one of the biggest challenges he has faced so far.