Sophon uses Universe 647’s control system to move the mini-universe’s exit through the great universe in search of a survivable destination. After three months and tens of thousands of jumps, the best option is a habitable but dangerous world rated three. Cheng Xin and Guan Yifan accept the risk and prepare to leave.

Before departing, Cheng Xin, Guan Yifan, and Sophon begin returning Universe 647’s mass to the great universe, answering the Returners’ call. Robots remove the soil, trees, metal floor tiles, and machinery, revealing a compact Trisolaran spaceship hidden beneath the floor. The ship is designed for three human occupants, equipped with fusion and curvature drives, ecological support, and hibernation equipment.

As the dismantling continues, gravity disappears, the artificial sky is taken apart, and the ecological systems shut down. Air, water, fog, and debris are vented through the door, transforming the once pastoral mini-universe into a dark, starless space. Cheng Xin and Guan Yifan put on space suits and briefly explore the one-cubic-kilometer universe’s closed geometry, seeing endless repeated images of the remaining objects.

The house, sun, robots, and almost all remaining structures are sent through the door. Sophon prepares a metal box to remain behind: a message in a bottle containing the stored memories of Earth and Trisolaran civilizations, intended for whatever new universe might be born after the next big bang. Cheng Xin and Guan Yifan imagine that many other civilizations may be leaving similar records behind.

Cheng Xin asks to leave five kilograms of matter in Universe 647: a sealed miniature ecological sphere she and Sophon made, containing water, fish, algae, grass, and a tiny sun. Guan Yifan agrees, though he privately reflects that even tiny losses of mass may matter to the universe’s fate, while also recognizing that many mini-universes will likely keep far more. Cheng Xin, Guan Yifan, and Sophon board the ship; Sophon, now dressed as a warrior, promises to protect them, and the ship passes through the door into the great universe.

After their departure, only the message box and Cheng Xin’s ecological sphere remain in Universe 647. The bottle fades into darkness, while the little living world continues to glow: fish swim between floating water spheres, grass holds dew, and a tiny ray of sunlight shines into the empty mini-universe.