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.
Jon Pertwee
The Doctor
Katy Manning
Jo Grant
Nicholas Courtney
Brigadier Lethbridge-Stewart
Richard Franklin
Captain Mike Yates
John Levene
Sergeant Benton
Roger Delgado
The Master
Pik-Sen Lim
Captain Chin Lee
Raymond Westwell
Prison Governor
