Blake's 7 1980 S03E09

Episode: Sarcophagus. Avon, Cally and Vila investigates a derelict spacecraft. When they return to the Liberator, they find something else has also returned with them, and Cally finds herself under the influence of a alien woman that assumes Cally's appearance and requires the crew to transport her back to her home planet, and only Avon can stop her.