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.
Quest roles:
Richard Franklin(Mike Yates), Nicholas Courtney(Brig. Lethbridge-Stewart), John Levene(Sergeant Benton), Roger Delgado(The Master), Neil McCarthy(Thawn), William Marlowe(Lester), Patrick Godfrey(Major Cosworth), Johnny Barrs(Fuller), Matthew Walters(Main Gate Prisoner)