The thefts of electronic components and secret plans have been happening both at night and in broad daylight and with the Doctor confined to sick bay there is little the Brigadier can do. When the Doctor recovers, he inspects the site of the most recent theft and realises that he and his friends are up against something distinctly non-human as heavy vehicle tracks have been found in the vicinity of the supposedly secure buildings which housed the components. In a later incident, the perpetrator actually drills through the ground to gain access to a heavily armed vault at Emmett Electronics. 

It is Sarah who first meets the culprit, a massive robot invented by one Professor J.P. Kettlewell while he was working for the Think Tank, a government installation involved in developing emerging technologies. The robot has been re-programmed under the direction of the director of Think Tank, Miss Hilda Winters, 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 obtain the computer codes controlling the nuclear weapons of the world's leading powers. In this way, the SRS hope to hold the world to ransom unless their demands for a purer way of life are met. 

Kettlewell, although claiming no knowledge of Miss Winters's activities, is actually a party to them, and is ultimately killed by his creation when he realises his error and tries to stop the Robot from wiping out a group of UNIT troops. 

The Robot suffers an electronic/mental breakdown upon killing its creator, and this coincides with the Brigadier's attempt to destroy the behemoth with the disintegrator gun. Unfortunately this causes the robot to grow and the now gigantic creature goes on the rampage through the UNIT troops, protecting only Sarah Jane with whom it feels an affinity. The Doctor manages to save the day by brewing up a virulent metal virus described in Kettlewell's notes, and using it to eat away the towering titan until there is nothing left but a small heap of decaying rust, which itself is soon destroyed by the virus. 

With this problem sorted out, the Doctor, now fully recovered following his change of appearance, decides that the time has come to travel once more. He persuades Sarah Jane and UNIT medic, Surgeon-Lieutenant Harry Sullivan, to join him. The Brigadier is less than pleased to see the TARDIS dematerialise, as the Doctor has been invited to an audience at the Palace.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}