Halo reaches Yun Tianming’s star system in only fifty-two hours from Cheng Xin and AA’s perspective, though 286 years pass in the Solar System’s frame. Cheng Xin remains numb with grief and guilt despite AA’s arguments that Cheng Xin could not have saved humanity. The ship identifies two planets: barren Planet Gray and life-bearing Planet Blue, whose blue coloration comes from vegetation rather than oceans.

Halo follows an old Crisis Era landing beacon to Planet Blue, where Cheng Xin and AA meet Guan Yifan, a Deterrence Era astronomer from Gravity who has been in hibernation for centuries. Guan Yifan explains that Blue Space and Gravity are now museum relics, Galactic humans have founded several worlds, and Planet Blue is unsettled because it lies near dangerous extraterrestrial routes. He also tells them the two-dimensional Solar System is now invisible except through gravity, and that little is known about Yun Tianming or the First Trisolaran Fleet.

Cheng Xin, AA, and Guan Yifan spend a day on Planet Blue, observing its mobile plant life and preparing to leave for a Galactic human world. AA becomes visibly attracted to Guan Yifan, while Cheng Xin accepts that her hope of meeting Yun Tianming may have been only a dream. During the night, monitoring satellites report that five unidentified spacecraft briefly landed on Planet Gray, so Guan Yifan and Cheng Xin travel in Hunter to investigate while AA remains with Halo.

On the journey, Guan Yifan explains the new human worlds, lightspeed travel, black domains, doomsday ships, and the darker truth of cosmic war. He says advanced civilizations weaponize physical laws themselves, especially dimensions and the speed of light; dimensional collapse never stops, and attackers can survive by transforming themselves into lower-dimensional beings. He describes the universe as a war-damaged ruin, perhaps once ten-dimensional with near-infinite lightspeed, and says the universe is dying.

At Planet Gray, Cheng Xin and Guan Yifan find five black death lines: zero-lightspeed curvature trails left by extremely advanced ships, possibly Zero-Homers trying to reset the universe. Guan Yifan explains that if disturbed, the death lines can rupture and spread into a system-wide reduced-lightspeed region, so they hurry back toward Planet Blue without using curvature propulsion. As Hunter returns, AA radios that Yun Tianming has arrived on Planet Blue and brought Cheng Xin a larger gift, but the reunion is cut off before Cheng Xin can see him.

As Cheng Xin and Guan Yifan descend by shuttle, the death lines rupture, lowering the speed of light across the system and killing ordinary electronics. The shuttle becomes trapped in a lightspeed orbit around Planet Blue inside a new black domain, with time outside passing vastly faster than inside. Guan Yifan uses analog emergency gear, neural computers, and hibernation drugs to give them a chance to survive until control systems reboot; before they sleep, he reassures Cheng Xin that she did not destroy humanity and that love was not wrong.