|Followed by||The Armageddon Inheritance|
It was later republished in the Empire From The Ashes compendium.
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 55,000+ year old technologically advanced multi-star system empire), the Dahak, led by its Chief of Engineering, the ambitious and psychopathic Captain Anu. Anu's ostensible reason for mutiny is to lead his followers to refuge on some remote planet where presumably the genocidal wrath of the "Achuultani", a mysterious alien race that periodically exterminates all intelligent life it can find, and which has destroyed the previous three Imperiums, will pass over them.
The loyal crew is taken by surprise, and unable to defeat the mutineers. 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 extremely deadly gases and compounds; this action will force both mutineers and loyalists to the lifeboats, and the vessel will then, acting on its other orders from the captain, allow back in the Dahak only the loyal crew and blow the disloyal members out of space. Red Two unfortunately entails the death of the captain as well. Because of his death, he is unable to command Dahak to destroy the mutineers as they leave aboard warships, not lifeboats, and the captain is equally unable to undo Anu's systematic sabotage of the power generators, intended to kill Dahak by starving it of power and thereby rendering it open to conquest by Anu's forces.
Unfortunately for Anu, Dahak's computer systems catch the sabotage before it utterly wrecks all the power plants, but the damage is so severe that it is forced to cease all communications and non-necessary expenditures of power. Indeed, the damage takes decades to repair—by which point none of the loyal crew is still alive or able to contact the Dahak. Outnumbered and outgunned by the mutineers, the loyalists have been systematically sought out and exterminated. This places the ship in a dilemma in which it cannot return to the Imperium as it has been ordered to, but nor can it exterminate the mutineers as other, equally important, orders dictate. This impasse lasts for approximately 50,000 years, until the Earthling's 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.
His mission is hijacked by Dahak and his death is faked; had MacIntyre returned with his data, Dahak’s cover would have been blown. While aboard, Dahak (the AI, not the vessel proper) explains the situation to MacIntyre, and prevails upon him to, as a descendant of the loyalists, become Dahak’s newest captain, and to quickly exterminate the mutineers—quickly, because Imperium installations are being methodically destroyed, sure signs of the beginning of the latest Achuultani incursion. MacIntyre reluctantly accepts. The first step to making him the true captain is to massively revamp his body surgically, granting him superhuman resilience, speed, and strength, in addition to the built-in electronics granting matchless control of Imperium technology.
While Dahak has known for thousands of years where Anu's forces have bunkered up—under the South Pole in Antarctica—their base is protected by extremely strong force fields, force fields so strong that to penetrate them and destroy the base would require Dahak's heavy weaponry, which would inevitably kill a significant percentage of the human population of Earth; an untenable action, to say the least.
MacIntyre returns to his old 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. It initially succeeds, but when he and Sean attempt to contact the agent, they discover their scan of the space program building was detected, and that it was a trap. MacIntyre and Sean fend off some of the mutineers (at the cost of Sean's life), but MacIntyre is rescued by an acquaintance, who sends him through a tunnel where he is captured by another group of mutineers.
This group, led by former missile tech Horus, was a dissident splinter faction of Anu's, which turned against him after the mutiny. Despite supporting Anu during the mutiny itself, Horus and his crew committed a double mutiny against Anu and fled into hiding on Earth. Once they reached Earth, they entered stasis so that the crew would be able to survive however long it would take for civilization to reappear on Earth (Anu at the time enforced primativism). Now, with civilization and technology emerging on Earth, his group has begun a passive, behind-the-scenes war against Anu. Because they are heavily outnumbered in weaponry, they have been forced to always play it very carefully. As a result, the crew of the battleship has created a huge network of humans, many of whom are descendants of Nergal's crew. However, the arrival of the MacIntyre means that the end has begun, for Dahak has at last taken a hand in the game. Eventually, this group and its battleship, the Nergal, joins MacIntyre, and they embark on a grand plan to destroy Anu: first, they rapidly and effectively destroy a number of important installations that Anu's terrorists and other forces are based out of (convincing Anu to withdraw all of his important personnel back to the main base), then they have their agents inside the Antarctica base steal the codes to gain access for them; finally, they fake a major defeat, and when Anu relaxes, certain that they were cowed and hurt, their now-at-liberty agents send them the codes and they launch a full assault, backed up by Dahak's orbital weaponry.
The assault costs them dearly, but Anu is killed and his forces captured or dead. With the full revelation of Dahak’s power, the world's governments have little choice but to submit to the Planetary Governor MacIntyre. However, Colin has little time to unify the world, because the Achuultani draw ever nearer, and the Imperium is silent, even when Dahak’s communication systems are repaired. At the end, MacIntyre leaves the world under the care of old Horus, and departs for the nearest Fleet Imperium base, hoping to call upon Imperial assistance.
Concept and creation
Author David Weber says the genesis for this book began with a question: "Assume that Earth doesn't actually have a Moon, but rather a giant starship disguised as our Moon which has been there for at least 50 or 60,000 years. Where did it come from, why did it come here, and why hasn't it left?" Weber says the answer to those questions built the foundation for this book and its sequels.