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In the fourth episode of the six-part story "The Mutants," the Doctor (Jon Pertwee) catches up with Solonian rebel Ky (Garrick Hagan), who has been framed for murder by the dictatorial Marshal (Paul Whitsun-Jones) of the orbiting Earth Skybase. Realizing that it is Ky to whom he is to deliver a secret package from the Time Lords, the Doctor completes his mission -- thus setting in motion the evolutionary process that will free the planet Solos from the Marshal's iron grip. But this story is far from over, as the Doctor will learn in the remaining two installments. Written by Bob Baker and David Martin, "The Mutants, Episode 4" first aired on April 29, 1972.