On Blue Space, Chu Yan shows Morovich and Guan Yifan how to find and enter warped points, small regions where three-dimensional space opens into a four-dimensional fragment. Inside, Morovich and Guan experience an overwhelming view of Blue Space and the surrounding universe: walls, sealed compartments, machinery, bodies, and space itself are all exposed at every level. Chu Yan explains that this four-dimensional access let Blue Space sabotage the Trisolaran droplets and capture Gravity by attacking three-dimensional objects from a higher-dimensional vantage.

After returning to three-dimensional space, Chu Yan and Guan describe the fragment as a four-dimensional bubble intersecting the three-dimensional universe. Gravity's equipment expands the investigation, and the crews discover many sealed four-dimensional objects, apparently artificial and inactive. The most striking is a giant ring-shaped object, the first true four-dimensional artifact humanity has observed, which appears to be an abandoned ruin or vessel.

As micrometeoroid strikes increase and threaten both ships, Chu Yan decides to leave the fragment for survival's sake. Guan argues that the Ring may contain vital knowledge, so a risky exploratory trip is approved. Guan, Lieutenant Zhuo Wen, and Dr. West travel by pinnace through four-dimensional space, nearly collide with the Ring because distance is unreliable there, and establish communication after sending mathematical signals and then the Rosetta System.

The Ring identifies itself as a tomb: once a spaceship, now dead, speaking as the remnant of its creators. It says the four-dimensional space is like a drying puddle after a vanished sea, and that other dead or dying ships have gathered there. Its warning links dimensional decay to the dark forest, stating that those who dried the sea moved from one dark forest to another; it urges the humans, fragile thin pictures, to leave. Before falling silent, the Ring asks for Guan's small ecological sphere containing a fish, and Guan gives it away.

Blue Space and Gravity retreat from the shrinking fragment. The crews observe four-dimensional matter decaying into three-dimensional luminous lines, then watch the Ring itself collapse into an enormous glowing line and dust belt, destroying its hidden four-dimensional structure. Guan and the science officers conclude that macroscopic higher dimensions naturally decay into lower dimensions, with the lost dimension curling into the microscopic realm.

After gathering limited materials from the Ring's remains, the crews decide their future. A temporary council allows everyone to choose between returning to the Solar System in a hibernation ark or continuing into deep space aboard Blue Space and Gravity. Only about two hundred choose to go back; the rest continue outward, accepting exile and uncertainty as the two ships carry seeds of human civilization toward a new target star system.