The "Doctor," still lacking his dematerialization equipment, along with his companion "Jo," are called to a correctional facility housing dangerous criminals to look into problems involving the intimidating "Keller" machine. This device is supposed to replace executions by absorbing the negativity of prisoners, essentially turning them into docile individuals like "Barnham." However, the machine has malfunctioned, resulting in a disturbance at the prison. Simultaneously, the "Brigadier" finds himself occupied with security arrangements for an initial global peace summit; when the Chinese representative dies, the "Doctor’s" focus shifts, leading him to suspect that these incidents are connected and that the "Master" might be orchestrating a scheme to utilize the collected evil within the "Keller" as revenge against both the "Doctor" and humankind. I've never been particularly fond of the stories set on Earth, and even with a complete cast of UNIT personnel, this one doesn’t rise above an average espionage story featuring infrequent special effects. Pertwee embraces the tone of it all wholeheartedly, Delgado performs his characteristic over-the-top villainy adequately, and there's a respectable group effort overall; however, the plot feels stretched thin, and while the writing is enjoyable at times, it has difficulty maintaining this breezy narrative across its six episodes. It’s perfectly acceptable viewing, but I really hope they return to adventures in space with those classic TARDIS stories involving aluminum foil, papier-mâché, and green spandex soon.

Source on TMDB