The TARDIS materialises in an Earth starship trapped in orbit around the planet Sense-Sphere by the Sensorites, aliens with telepathic powers who blame the crew of a previous Earth-ship for a deadly plague. Two of the ship's personal, the Captain (Lorne Cossett) and Carol (Ilona Rodgers) are under the partial/occasional control of the aliens and a third, John, appears to have been driven mad by the 'voices in his head'. The Doctor attempts to convince the highly suspicious Sensorites that he and his companions (Ian, Barbara, and Susan) mean them no harm (and may be able to stop the plague) but becomes embedded in a power struggle involving an unethical and paranoid 'City Administrator' who sees the governing Elders' growing acceptance of the strangers as evidence of their weakness and as a threat to the survival of the Sensorite race. The six-part serial is a bit stretched, notably the amount of time devoted to the renegade Administrator's treasonous machinations but, as vintage Doctor Who stories go, the plot is moderately interesting and the aliens and sets well done (for the budget and era). As usual, BBC parsimony is evident in the frequent muffing of lines that would have led to retakes in a more prosperous production. The premise that all Sensorites look the same, which allows convenient impersonations, is a bit of a stretch. Perhaps to Sensorite eyes that is the case but, since it is apparent to us viewers that the 'aliens' vary in size and build (one has a noticeable pot belly), the Gallifreyan and human characters should have been able to spot a counterfeit. Also, the Sensorites all have recognisably different voices - the serial would have been more clever if some effort had been made to match body types and to have all of the Sensorites speak through a single voice-over. All-in-all, not great but fans of Hartnell's interpretation of the venerable Timelord and of the first generation of companions should enjoy this reasonably imaginative adventure. Interesting to hear Susan reminisce about the burnt-orange skies and silver-leaved trees of her home world (presumably Gallifrey). *Score and comments pertain to all six parts.