The Doctor and Jo head to Stangmoor Prison to investigate the Keller Machine, which drains the evil impulses from prisoners' minds, while UNIT are handling security at the first world peace conference.

The Doctor is recalled by UNIT to help investigate Cheng-Tiek's murder but is unaware that Chin-Lee is under the control of the Master.

Realising Chin-Lee has been using the power of the Keller Machine to carry out assassinations, the Doctor heads back to Stangmoor, only to find the Master has teamed up with Mailer to stage another riot.

With the Doctor and Jo being held prisoner, the Master convinces Mailer to help him steal the Thunderbolt.

The Brigadier has recaptured Stangmoor but the Master still has control of the Thunderbolt and the Doctor must find a way to stop him and destroy the Keller Machine.