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In the conclusion of the seven-part story "Inferno," the Doctor (Jon Pertwee) is able to escape the destruction of the alternative-universe earth, brought about by a drilling project designed to locate new energy sources. Returning to his own universe, the Doctor races to prevent a similar disaster befalling the "real" earth. But in doing so, he may be profoundly altering the future of humankind in other ways -- and not necessarily for the better. Caroline John makes her farewell appearance as Dr. Liz Shaw. Written by Don Houghton, "Inferno, Episode 7" first aired on June 20, 1970, as the final episode of Doctor Who's seventh season.