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 c... read the rest.