When the detective in charge of investigating a series of bank robberies starts to get too close to the culprits, they set up a blackmail scheme to warn him off. But when the crooks begin to fall out with each other, the police learn the truth.

After her father's death, Maud, a young heiress, is sent to live with her Uncle Silas. She finds her new guardian living in a gothic nightmare of a mansion, and soon realizes that he wishes to murder her and claim her inheritance.

The TARDIS arrives in London in 1966 and the First Doctor and Dodo visit the Post Office Tower. There they meet Professor Brett, whose revolutionary new computer WOTAN (Will Operating Thought ANalogue) can actually think for itself and is shortly to be linked up to other major computers around the world — a project overseen by civil servant Sir Charles Summer.