Kemlo, with two other boys from Satellite K, are allowed to accompany a small research team setting off to reconnoiter Deimos, a moonlet of Mars.
The research ship has a smooth journey until it encounters a fierce astral storm, but eventually lands safely on Deimos - where some peculiar magnetic forces put all the instruments out of action.
From then on the suspense is terrific. Kemlo and his friends find their way barred by a moving wall of lighted shapes and, scattered, are apparently attacked by transparent figures etched in vivid light. Up to the very end the doubt remains: will the power of the Martian Ghosts triumph or be broken?