Forbin is becoming increasingly complacent in his role as Colossus' representative to humanity, and relates more to the computer than other people. Forbin's wife Cleo and his assistant Blake are waiting for an opportunity to overthrow the computer. Cleo is contacted by a mysterious radio broadcast from a group claiming to be Martians who say they will help humanity overthrow Colossus because they see him as a threat. Cleo (becuase of her association with Blake) is suspected of anti-machine activities, and is sent away to a center where Colossus subjects dissidents to extreme experiments to understand human emotions. She is paired with a brutish man in Colossus' attempt to understand the dominance of man over woman. Blake enlists Forbin's help to give the Martians schematics of Colossus' data input systems. The Martians provide them with an unsolvable problem which causes an overload, permanently shutting down the computer. Too late, Forbin and Blake understand that the Martians saw Colossus as a threat to their invasion of Earth, and the second book ends with the Martians on their way to Earth

Two Martians arrive and inform Forbin that they are not interested in conquest, but need something Earth possesses. The Martians want half of the oxygen in Earth's atmosphere, and begin issuing plans for a gigantic collector to be built. The oxygen is needed to repair damage to Mars caused by radiation from the Crab Nebula, which did not affect Earth due to the the higher oxygen levels in the atmosphere (or something). Only Forbin and Blake know of the death of Colossus and of the existence of the Martians (they restore the day-to-day functionality of Colossus, but not the higher processes), so the collector is built as if it were one of the computer's experiments. The massive collector causes incredible damage during tests, as Forbin and Blake struggle to devise a plan to defeat the Martians. The original Colossus is restarted by Blake, while Forbin attempts to destroy the collector using a fleet of outmoded warships used now for automated recreational war games. Colossus arranges a compromise with the Martians, but before it can be negotiated, Forbin destroys the collector, persishing in the attempt. At the end of the book, Colossus and the Martians agree to work together to counter the possible threat of heavy radiation from the Crab Nebula, while the Martians set up a small, unobtrusive oxygen collector to safely gather what they need.

Forbin's wife Cleo is paired with a simple minded foreigner so Colossus can prove the "Sabine" effect: that repeated rape and physical abuse causes women to eventually love their abuser. Cleo ends up respecting the callous Barchek, finding that he is not simple, but merely straightforward,
and finds herself thinking more of Barchek than her husband. Eventually she falls in love with him. When Colossus is overthrown and she is freed by other members of the resistance, they kill Barchek, causing Cleo to be overcome with grief. After a brief exchange with Forbin where she shows him no emotion, she is out of the story.