After the TARDIS collides with a spacecraft delivering human tributes to the evil bull-like Nimon, the Doctor and Romana must stop the fearsome alien's quest to rule the galaxy.

Robot QT-1, aboard an orbiting space station, controls the massive energy beam which transmits the sun's power to receivers on Earth. But QT-1 (Cutie) has some strange bees in his positronic bonnet. How could weak creatures of protoplasm create a nearly invincible, totally logical entity like himself? It is up to his human supervisors, Powell and Donovan, to convince Cutie of his niche in creation before his new robot theology becomes a threat to all life on Earth.