[3] The narrator, 16-year-old Jeremy Grant, who has been recently orphaned, travels from Australia to Scotland to stay with his scientist uncle, Dr Lachlan McKinnon, at his estate, Inverard.

He discovers that the irascible McKinnon and his colleagues, including Swedish Professor Lars Bergman, American engineer Spike Stranahan, science student Janet Campbell, Cockney housekeeper Madge Smith and engineer Kurt Oppenheim, are building an atomic-powered spaceship to travel to Hesikos, the wandering 'lost planet' which is now within a few days' flight of Earth, and which is known to have near-Earth gravity and a breathable atmosphere.

It transpires that Oppenheim is a spy and saboteur working for a rival expedition, led by Professor Hermanoff, from an Eastern European country.

McKinnon is worried about overloading the spaceship, and at the last minute, unbeknown to the rest, he leaves the ship to return to his own, hoping to survive the cold until a relief expedition can be organised.

Safely back on Earth, Grant and Campbell pick up a Morse code radio message from Hesikos confirming that McKinnon is alive.