A troop of British commandos return from a covert mission suffering from mysterious symptoms and horrific burns. Dr. Adam Royston, an atomic scientist, suspects lethal radiation, and must track down the source of this menace before it overtakes humanity. A desolate area of Scotland is threatened by the cyclical appearance of a giant radioactive blob of mud that arises from the depths of the Earth to consume everything in its path. Good, atmospheric sci-fi horror from the masters at Hammer.