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.

The Doctor is forced to help the Master control the mind parasite while Yates finds out where the Thunderbolt has been taken only to be captured by the Master's men.

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.