The Doctor realises the Master is trying to harness the power of Kronos, last of the Chronovores, but is unaware Percival is harbouring him at the institute.
Quest roles:
Richard Franklin(Mike Yates), Neville Barber(Dr. Cook), Nicholas Courtney(Brig. Lethbridge-Stewart), John Levene(Sergeant Benton), Roger Delgado(The Master), Wanda Moore(Dr. Ruth Ingram), Ian Collier(The Renegade (voice)), John Wyse(Dr. Percival), Donald Eccles(Krasis), Aidan Murphy(Hippias), Keith Dalton(Neophite), Barry Ashton(Kemp)