Professor Emil Keller has created a machine that can pacify even the most dangerous of criminals. But when the Doctor and Jo arrive at Stangmoor Prison for a demonstration, things start to go horribly wrong - especially when they discover that the Doctor's old enemy the Master is responsible for the machine.
The Doctor
Jo Grant
Brigadier Lethbridge-Stewart
Captain Mike Yates
Sergeant Benton
The Master
Captain Chin Lee
Prison Governor
Dr. Roland Summers
Professor Kettering
Corporal Bell
George Patrick Barnham
Arthur Linwood
Chief Prison Officer Powers
Senior Prison Officer Green
Prison Officer
Prison Officer
Prison Officer
Prison Officer
Mailer
Vosper
Fu Peng
Senator Alcott
Charlie
Major Cosworth
Fuller
Main Gates Prisoner
Prisoner (uncredited)
