The book's premise is that the Moon is a massive space ship controlled by a self-aware computer that wants its rightful crew back aboard.

The book begins with a prologue recording a mutiny aboard the planetoid-sized Utu-class starship of the Fourth Imperium (a more-than-55,000-year-old technologically advanced multi-star system empire), the Dahak, led by its Chief of Engineering, the ambitious and psychopathic Captain Anu and Commander Inanna.

Anu's reason for mutiny is to lead his followers to refuge on a planet where presumably the genocidal wrath of the "Achuultani", a mysterious alien race that periodically exterminates all intelligent life it can find, which has also destroyed the previous three Imperiums and the dinosaurs, will pass over them.

Faced with no choice, the captain orders Dahak to execute "Red Two Internal"—a command which will flood the entirety of the interior of the vessel with deadly substances; this action will force mutineers and loyalists to the lifeboats, and the vessel will then, acting on other orders from the captain, allow back in the Dahak only the loyal crew and blow the mutineers into space.

This lasts for approximately 50,000 years, until the Earthlings' early space program sends up one Lieutenant Commander Colin MacIntyre to map the dark side of the heavenly body Dahak had camouflaged itself as—the Moon—as a "dress rehearsal" for a similar trip scheduled for Mars.

MacIntyre returns to his home to renew contacts with his elder brother, Sean, and to enlist him in a scheme to discover the mutineers' agent in the space program.

However, Colin has little time to unify the world, because the Achuultani draws ever nearer, and the Imperium is silent, even when Dahak's communication systems are repaired.

Finally, MacIntyre leaves the world under the care of old Horus, and departs for the nearest Fleet Imperium base, hoping to call upon Imperial assistance.