The Doctor and Jo Grant investigate the Keller Machine at Stangmoor Prison, which is being used to remove any malicious elements of prisoners’ personalities. 

Meanwhile, the Brigadier and UNIT are being stretched as they are responsible for security for the World Peace Conference, as well as having to deal with the decommissioning of a Thunderbolt Missile.  Things only get more complicated for the Doctor and the Brigadier when they discover that the Master is behind the Keller Machine.

Read More The Mind of Evil