Doctor Who and the Brain of Morbius

Why do so many spaceships crash-land on Karn, a bleak, lonely and seemingly deserted planet? Are they doomed by the mysterious powers of the strange, black-robed Sisterhood, jealously guarding their secret of eternal life? Or does the mad Dr. Solon, for some evil purpose of his own, need the bodies of the victims — and more especially, the body of Doctor Who?