The Mutants

From http://www.bbc.co.uk/doctorwho/classic/episodeguide

The Time Lords send the Doctor and Jo on a mission to deliver a sealed message pod to an unknown party aboard a Skybase orbiting the planet Solos in the 30th Century. Solos is due to gain independence from Earth's empire, but its Marshal is determined to prevent this. He arranges the murder of the Earth Administrator and, with his chief scientist Jaeger, plans to transform Solos's atmosphere into one more suited to humans.

Ky, a young Solonian leader, is falsely accused of the murder, and flees to the planet, taking Jo with him. The Doctor follows and joins them in an old thaesium mine. Ky turns out to be the intended recipient of the message pod, which opens automatically for him. Inside are stone tablets carved with ancient inscriptions.

The Doctor's party then meet Sondergaard, a human scientist leading a hermit-like existence in the mine while searching for a cure for the mutating disease that afflicts the Solonians. The Doctor and Sondergaard decipher the inscriptions, deducing that the mutations are part of a natural life-cycle in which the thaesium radiation plays a vital role.

The Doctor retrieves a crystal from a cave where the radiation is concentrated and returns to the Skybase to analyze it. He is recaptured by the Marshal and, with his friends held hostage, is forced to perfect the machine with which Jaeger plans to transform Solos. Sondergaard meanwhile gives Ky the crystal, which turns him first into a mutant, and then into an ethereal super-being - the ultimate stage of the Solonians' life-cycle. Jaeger is killed when the Doctor sabotages his machine, and the Marshal is vaporised by Ky.

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