The newly-regenerated Doctor helps UNIT battle a sentient robot being manipulated by a corrupt scientific organization.
The TARDIS arrives on the planet Terra Alpha, where the Seventh Doctor and Ace discover a society in which sadness is against the law - a law enforced zealously by the brightly uniformed Happiness Patrol. The planet is ruled by Helen A with the aid of her companion, Joseph C, and her carnivorous pet Stigorax, Fifi.
The arrival of a mysterious comet heralds impending danger from enemies both old and new. As Ace helps the Doctor defend Earth, she is confronted with a dangerous question..."Doctor Who?"
