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.

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.