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.
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 execute these miscreants with a process that soaks up all their evil and leaves them like the placid "Barnham" (Neil McCarthy). Well, needless to say, the thing has gone a bit wonky and there's a bit of a riot broken out at the establishment. Meantime, the "Brigadier" (Nicholas Courtney) has his work cut out for him organising the security at the first world peace conference. The Chinese delegate is killed and the "Doctor" is diverted from his original task - only to discover that these events might have something in common and that the "Master" (Roger Delgado) might be behind a cunning plot to use all the evil stopped up in the "Keller" to avenge himself on both the "Doctor" and on us pesky humans. I was never really as much of a fan of the Earth-bound adventures and despite a full suite of UNIT characters this one isn't really much better than an average spy thriller with some sparingly used special effects. Pertwee enters into the spirit of this enthusiastically, Delgado delivers his usual brand of hammy megalomania well enough too and it's a decent enough ensemble effort, but it's too thinly strung out and the writing - though quite entertaining a times - struggles to sustain this light-weight outing across six episodes. It's all watchable enough, but please can we get back into the TARDIS for some off-world tin foil, papier-mâché and green lycra stories soon, please?