Mikhail Vadimov dies after his car crashes off the Alexander Hamilton Bridge into the Harlem River. An autopsy shows that leukemia caused retinal hemorrhages that led to the accident. Cheng Xin grieves him as a generous mentor, but the intelligence side of the PIA focuses on the loss of his usable brain, and Cheng Xin begins to suspect that his illness may have been deliberately induced by radiation.

Vadimov’s death deepens Cheng Xin’s guilt over Yun Tianming, who has been moved with six other candidates to a secret base for final Staircase Program testing. Remembering Tianming’s loneliness in college and dreaming of his brain trapped near a black hole, Cheng Xin realizes that recommending him may have stolen his chance at hibernation and future treatment. She asks Thomas Wade to remove Candidate #5, but Wade says Tianming has performed best in the tests and argues that Tianming’s isolation, adaptability, and creativity make him unusually suitable.

At the UN oath ceremony, the candidates are expected to pledge eternal loyalty to humanity before leaving the Solar System. When Tianming’s turn comes, he refuses, saying he feels like a stranger in the world and will judge his loyalty after seeing Trisolaran civilization. Cheng Xin is horrified, but Wade treats the refusal as proof that Tianming can deceive the enemy because even humans believe him. After another candidate, Joyner, also refuses and then dies before her brain can be preserved, Tianming is chosen for the mission.

When Tianming’s condition suddenly worsens, Cheng Xin rushes to Westchester Medical Center but arrives after surgeons have already removed and frozen his brain. Wade reveals just before she enters that Tianming was the one who gave her the star, devastating her. A white-haired brain surgeon offers Cheng Xin a slender hope: if Trisolarans can revive the brain, they might clone Tianming’s body from its genetic information and restore him.

Cheng Xin asks Wade to include seeds in the Staircase capsule so Tianming might one day eat after being restored. Wade refuses the extra mass, but then uses Cheng Xin’s attachment to Tianming to keep her in the PIA, offering to send her into hibernation as someone who knows him. The Staircase spacecraft launches and reaches 1 percent of lightspeed after hundreds of nuclear detonations, but a cable failure curls the sail and sends the capsule off course, likely missing the Trisolaran fleet forever. With no immediate reason left to follow Tianming, Cheng Xin still enters suspended animation as a future liaison for the program, comforted by the thought that she, too, will cross centuries in darkness.