Death's End

Cheng Xin, having succeeded Luo Ji as the second Swordholder responsible for enforcing dark forest deterrence against Trisolaris, freezes when Trisolaris cripples Earth's broadcast stations and gambles that she will not retaliate. She is right: Cheng Xin cannot bring herself to trigger mutual annihilation, and Trisolaris moves to renew its invasion, offering humanity a token reservation in Australia. Salvation comes from an unexpected quarter: the crew of the human starship Gravity, together with the crew of the fugitive ship Blue Space (who had escaped into the fourth dimension), foil a Trisolaran attempt to destroy Gravity's own broadcast capability. Believing Earth already lost, the combined crews broadcast Trisolaris's location to the galaxy as an act of vengeance, and Trisolaris is annihilated by an unseen, far more advanced civilization. The surviving Trisolaran fleets, now aware that Earth's proximity to the broadcast makes it a target too, abandon their invasion.

The reprieve is temporary. Yun Tianming, whose brain had been captured and revived by the Trisolarans decades earlier, is permitted to pass Cheng Xin a coded message disguised as fairy tales, containing survival strategies for humanity. From this, Earth pursues bunker cities in the shadow of gas giants, and, more controversially, lightspeed spacecraft technology, which is ultimately outlawed as "escapism." Thomas Wade, the ruthless intelligence chief who has reappeared throughout the story, secretly builds a working lightspeed engine anyway; when Cheng Xin, awakened from hibernation, refuses to authorize its use, Wade is executed for the illegal program. Decades later, a hostile alien civilization launches the dark-forest strike humanity feared: an attack that collapses the three-dimensional Solar System into two dimensions. Without lightspeed ships, almost all of humanity is killed instantly. Cheng Xin and her aide AA, roused from hibernation in time, escape aboard a ship secretly fitted with Wade's illegal engine, after a final visit to the dying Luo Ji on Pluto.

Traveling near lightspeed, Cheng Xin and AA experience only a fraction of the 286 years that pass before they reach Planet Blue, orbiting Cheng Xin's own star, DX3906. There they find that the crews of Gravity and Blue Space have founded small interstellar human societies, and they meet Guan Yifan, a cosmologist from Gravity. Cheng Xin and Guan travel on to a neighboring world, Planet Gray, where they discover the seed of a "black domain," a region of artificially slowed lightspeed meant to signal harmlessness to the universe. The domain grows to swallow the entire star system, and the resulting time dilation strands Cheng Xin and Guan for what amounts to roughly nineteen million years relative to Planet Blue by the time they return.

In their long absence, Yun Tianming had visited Planet Blue and left behind a gift: Universe 647, a self-contained pocket universe built by the Trisolarans, existing outside the main universe's timeline. Cheng Xin and Guan Yifan settle inside this miniature, pastoral universe, hoping to outlast the death of the greater universe and witness its rebirth in a future Big Bounce. Their peace is broken by a message revealing that pocket universes like theirs are draining mass from the main universe, preventing it from ever collapsing and being reborn. Accepting a final moral responsibility to the cosmos itself, Cheng Xin and Guan Yifan choose to dissolve Universe 647, returning its mass to the greater universe, and the novel closes with the two of them, now among the last of humanity, setting off once more into the wider universe.