An expedition to Mars is decided on and Chris, Serge, Morrey and Tony are chosen to man it. Unlike their expedition to Venus, this is not a desperate last-minute venture; it is a sober, carefully planned affair. Chris and his friends have no reason to expect anything beyond the normal risks of space travel - except for the experiences of the Dutchman Van der Veen. He is the only man who has ever penetrated beyond the Le Prince layer, which blots out radio communication with the earth - and he returned in a state of mental collapse. When he hears of the new expedition he has another breakdown, and when at last he is able to describe his experiences he speaks of strange and terrifying voices that assailed him in outer space.
Will Chris and his friends also here these voices, and what will they find on Mars?