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, K1, 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 has a change of heart and tries to stop the countdown, but Jellicoe pulls a gun on him. Harry and Sarah have escaped their bonds and Harry knocks Jellicoe to the ground. Kettlewell holds the countdown while the other two open the doors. As they exit the bunker, K1 swings the disintegrator gun on them.
Sarah tries to convince K1 that Think Tank are evil. K1 struggles with the dilemma and fires the gun, disintegrating Kettlewell. With a wail, the robot cries out that it has killed its creator and collapses. With K1 apparently disabled, the Doctor and the UNIT troops make their way into the bunker. The robot then suffers an electronic mental breakdown and tries to activate the nuclear weapons.
As K1 exits the bunker, the Brigadier fires at it with the disintegrator gun. Instead of being destroyed, the robot grows to gigantic size. It picks up Sarah like a doll and heads towards the nearby village.
It places her on a rooftop as a pitched battle takes place between K1 and UNIT troops, while the Doctor races back with the virus. Driving by K1's feet in his roadster, Bessie, the Doctor throws the batch of virus at it. The virus instantly spreads over the robot, throwing its growth into reverse. It shrinks to doll size and then dissolves completely.
Figures by Black Tree, Tank by Corgi, K1 Robot by Character options.