The planet Karn is home both to a mystic Sisterhood, whose sacred flame produces an elixir of life, and to Mehendri Solon, a fanatical scientist who is using the remnants of spaceship crash victims to put together a new body for the still-living brain of the executed Time Lord criminal Morbius. When the Doctor and Sarah arrive on the planet, Solon decides that the Doctor's head is just what he needs to complete his work.