A newly regenerated Doctor joins UNIT in an investigation into the theft of top secret plans and equipment from supposedly secure premises. Sarah discovers that the raids have been carried out by a robot invented by scientist Professor Kettlewell while he was working for Think Tank, a body involved in developing emerging technologies.

The robot has been reprogrammed on the orders of Miss Winters, the director of Think Tank, and used to obtain the means for constructing a disintegrator gun with which the Scientific Reform Society - of which she is a leading member - can steal the computer codes controlling the nuclear weapons of the world's leading powers. In this way, the SRS hopes to hold the world to ransom unless its demands for a purer way of life are met.

Kettlewell is killed by the robot after he balks at Miss Winters' ruthlessness. The robot then suffers an electronic mental breakdown and tries to activate the nuclear weapons.

The Brigadier attempts to destroy it with the disintegrator gun, but this merely causes it to grow to gigantic proportions, following which it goes on the rampage.

The Doctor, assisted by UNIT medical officer Harry Sullivan, destroys it with a metal virus described in Kettlewell's notes.