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
CinemaSerf
Still without his dematerialisation circuit, the trapped "Doctor" (Jon Pertwee) and his assistant "Jo" (Katy Manning) are summoned to a prison for violent felons to investigate a mystery surrounding the rather menacing "Keller" machine. It's a sort of high-tech gadget that substitutes the need to ex...