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.
Social & External

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)
