London, 1966. The TARDIS materialises in the shadow of the newly-completed Post Office Tower in London and the Doctor senses a strange energy in the air. He instinctively knows that evil is at work nearby.

Posing as a scientist, the Doctor and his 'secretary' Dodo gain access to a suite at the top of the tower, and meet the driven Professor Brett. His life's work, the thinking computer WOTAN, is about to be linked up in a problem-solving network with many other machines around the world.

But the Doctor is concerned. How can WOTAN possibly know the meaning of the word TARDIS and about the Doctor's travels through time and space? What is the strange control that WOTAN can exert over humans via a mere telephone call? And what is the computer's link with the deadly robots being assembled in a Covent Garden warehouse? Soon, London will face an army of war machines, ruthlessly programmed to eliminate all who stand in their way...