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.
Quest roles:
Richard Franklin(Mike Yates), Nicholas Courtney(Brig. Lethbridge-Stewart), John Levene(Sergeant Benton), Dave Carter(Sergeant Duffy), Roger Delgado(The Master), Pik-Sen Lim(Captain Chin Lee), Raymond Westwell(Prison Governor), Michael Sheard(Laurence Scarman), Clive Scott(Linwood), Roy Purcell(President of the Council), Martin Gordon(Prison Officer), Neil McCarthy(Thawn), Eric Mason(C.P.O. Smedley), Simon Lack(Zadek), Bill Matthews(Prison Officer), Fernanda Marlowe(Corporal Bell), Barry Wade(Prison Officer)