For the first time Morrey, Serge and Tony make a space expedition without Chris, who has become Deputy Director of U.N.E.X.A.. Their dedtination is Pluto, which since its discovery in 1930 has always been thought the most distant of the planets. Now, however, the powerful instruments of the Lunar Observatory have detected a change in its orbit which suggests the existence of another planet beyond it. The task of Morrey and his crew is to learn more about this mysterious Planet X and also to try out a new form of propulsion which will send their ship through space faster than ever before.

The launching is a complete success, but as the crew are approaching Pluto they make a terrible discovery about their ship...