The Doctor is caught up in events at Stangmoor Prison, where Professor Keller’s new machine is said to be able to remove all evil impulses from the minds of the prisoners.
The Doctor and Jo watch a demonstration of the Keller Machine which can extract evil impulses from criminals.
A full scale riot takes place at the Prison and Jo is taken hostage.
The Master is behind the Keller Machine and uses it to recruit the rioting inmates at Stangmoor Prison.
The mind parasite within the Keller Machine becomes too powerful for the Master to control.
The Master forms an uneasy alliance with the Doctor so that between them they can subdue the Keller Machine.
The Keller Machine breaks free of the Doctor's restraining device. The Doctor turns it against the Master.
Third Doctor
Jon Pertwee
Alistair Gordon Lethbridge-Stewart
Nicholas Courtney
Jo Grant
Katy Manning
Mike Yates
Richard Franklin
Captain Chin Lee
Pik-Sen Lim
Prison Governor
Raymond Westwell
Dr. Summers
Michael Sheard
Professor Kettering
Simon Lack
Barnham
Neil McCarthy
Corporal Bell
Fernanda Marlowe
Linwood
Clive Scott
Chief Prison Officer Powers
Roy Purcell
Senior Prison Officer Green
Eric Mason
The Master
Roger Delgado
John Benton
John Levene
Mailer
William Marlowe
Vosper
Haydn Jones
Fu Peng
Kristopher Kum
Senator Alcott
Tommy Duggan
Charlie
David Calderisi
Major Cosworth
Patrick Godfrey
Fuller
Johnny Barrs
Main Gate Prisoner
Matthew Walters
Third Doctor
Jon Pertwee
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