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...
The boys go underground again, this time to rescue Ian, a spoiled billionaire's grandson who defied the rules of the Caves of Drach on Mallorca and went wandering off. Chris and his mates encounter an atmosphere that makes them giddy, and it's up to their leader to set them straight. They then discover Cenobia, a subterranean civilization, and Ian right in the middle of it. The Cenobians want everyone to stay because they don't want to be corrupted by the surface dwellers finding out about them.
Meanwhile, Whiskers shows up to lead another rescue team, and Cenobia is in danger of being discovered anyway.
The boys quarrel about sharing Cenobian tech with the Outer World, but Chris plots with the Cenobian leader to find a solution. So everyone else's memory gets wiped again, Ian is rescued, and Cenobia is safe. Although it's unclear if Chris will take the (red) pill at the end.