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
as The Doctor
Katy Manning
as Jo Grant
Nicholas Courtney
as Brigadier Lethbridge-Stewart
Richard Franklin
as Captain Mike Yates
John Levene
as Sergeant Benton
Roger Delgado
as The Master
Pik-Sen Lim
as Captain Chin Lee
Raymond Westwell
as Prison Governor
Michael Sheard
as Dr. Roland Summers
Simon Lack
as Professor Kettering
Fernanda Marlowe
as Corporal Bell
Neil McCarthy
as George Patrick Barnham
