As the Trisolar Crisis begins, Cheng Xin completes graduate school and joins a high-profile Long March rocket propulsion task force. Although the job promises advancement, Cheng Xin becomes disillusioned with chemical rockets, seeing them as obsolete and inadequate for true spaceflight or Solar System defense. When the United Nations forms new Planetary Defense Council agencies, Cheng Xin accepts a position at the PDC Strategic Intelligence Agency, or PIA, because it offers work aimed directly at the Trisolaran threat.

Cheng Xin arrives at PIA headquarters near the United Nations and meets Mikhail Vadimov, her boss at the Technology Planning Center. Vadimov warns Cheng Xin that PIA combines technical experts with intelligence veterans, whose profession depends on lies and betrayal. Cheng Xin then meets PIA Chief Thomas Wade, a cold and unsettling American whose first test of Cheng Xin is a brutal question meant to introduce the moral flexibility of intelligence work.

At PIA’s first all-hands meeting, Assistant Chief Yu Weiming urges staff members to balance national loyalty with loyalty to humanity, but the agency is already marked by distrust among nations and professional groups. After dismissing most of the room, Wade tells the spaceflight experts that PIA must launch a spy probe toward the Trisolaran Fleet. Wade insists the probe must reach one percent of lightspeed, arguing that sophons may prevent future breakthroughs, so humanity must begin immediately despite the apparent impossibility.

When the experts dismiss Wade’s demand as technologically unrealistic, Wade asks Cheng Xin for ideas. Cheng Xin proposes using a tiny probe with a radiation sail and detonating nuclear bombs placed in advance along its route, so the probe gains speed without carrying its own fuel. The experts initially mock Cheng Xin, especially Camille, but they fall silent when Cheng Xin explains the bombs would be distributed through space. Wade and the team recognize the idea’s potential and name it the Staircase Program.

During the following week, Cheng Xin works with Vadimov’s task force to coordinate independent feasibility studies by major space agencies. The studies show that the sail can be made lighter than expected, but the total probe must be reduced to only 200 kilograms to reach the required speed. Wade then reveals that the PDC has rejected the Staircase Program because its intelligence value seems negligible: the probe cannot decelerate, its sensors would be weak, sophons would expose the plan, and the Trisolarans likely use communication methods beyond human detection.

Although Wade admits the PDC’s objections are correct, he refuses to abandon the plan and orders the team to keep advancing until he says otherwise. Wade then escalates the concept by proposing that the probe carry a person in hibernation: a human representative inserted into the Trisolaran Fleet. The idea depends on the Trisolarans using their sophon-given knowledge to intercept and capture the probe, turning humanity’s transparency into a possible advantage.