Chris Godfrey, the famous astronaut, is on holiday in Majorca with his friends Morrey, Serge and Tony. They visit the fabulous underground Caves of Drachm and there they encounter Ebenezer Yates, an elderly and wealthy American who is greatly distressed because his grandson Ian has just disappeared. There is one cavern to which no one is admitted; armed soldiers guard its entrance, but Mr Yates knows that Ian was fascinated by this forbidden cave and he is sure that the boy has slipped in while the lights were switched off. The cavern is regarded with such terror that no one will talk about it, but Mr Yates finally discovers that before it was kept guarded a number of people had entered and none had ever returned. Nonetheless, the astronauts are determined to find Ian no matter what the danger...