On her maiden voyage to Venus, the space liner Cosmic Cloud encounters a mysterious voice speaking through the void. The language is alien-but it is realised that danger threatens. Hardly has a recording been sent to Earth for expert interpretation before a mysterious gong is heard in the space liner, followed by further nerve-shattering notes until, abruptly, the space liner is utterly destroyed. Eventually it is learned that a diabolical mechanism is buried somewhere on Venus, left behind by a long-vanished malign alien race, and at a time appointed it will destroy the whole Solar System. Three men and a girl travel to Venus in a desperate race against time to find and deactivate the doomsday device...