In the besieged Constantinople, emperor Constantine XI meets with a prostitute named Helena who promises to kill his enemy the sultan who is attacking the city. Skeptical, he gives her a test to return with the head of a prisoner, which she seemingly magically achieves but only returns with his brain. Unfortunately for the emperor, she is unable to kill the sultan since the higher dimensional fragment she used had left the Earth behind.
We’re introduced to Yun Tianming, an aerospace graduate who has late-stage terminal lung cancer. He decides that rather than spend what little money he has left over, that he will utilize the newly passed euthanasia law to take his own life.
Even after receiving a large sum of money from his old college classmate as repayment for his drink idea that made his classmate rich, he is not able to use that money towards his treatment, so instead he decides to anonymously buy DX3906, a remote, planetless star for his college infatuation, Cheng Xin utilizing the UN’s newly created “Stars Our Destination” project.
At the start of the Crisis Era, Cheng Xin, a aerospace PhD, joins the newly formed and secretive branch of the PDC called the Strategic Intelligence Agency (PIA) where they research ways to defeat the Trisolarans.
Initially dismissed because of inherent sexism, she ultimately proposes a plan to fulfil the PIA’s mission to accelerate a spy probe to 1% the speed of light to intercept the Trisolaran fleet, gaining the trust of her boss Vadimov and PIA chief Thomas Wade.
Her plan, dubbed The Staircase Program, would use bombs detonated in precise succession in space to propel a probe using a sail to the desired speed. During the research phase that started out on sending a hibernating person, to a frozen dead person, ends up being only a frozen brain to lower the weight enough to be able to achieve 1% the speed of light.
While searching for a suitable candidate, her classmate tells her about the terminal condition of her former classmate Yun Tianming, whom she believes would be a good candidate. The PIA works in secret with world governments to pass the euthanasia laws so that a candidate’s brain can be harvested to send into space.
Tianming’s attempt to conduct euthanasia is interrupted by Cheng Xin at the last moment, but he realizes it is only in a professional capacity, not out of love because he had gifted her a star as he initially thought. All the while Cheng Xin had no idea who had given her the star, which has little value other than sentimental. Tianming accepts the mission, despite his fear of space and the thought of eternal torture at the hands of the Trisolarans.
It is only as Tianming is about to die that Wade tells Cheng Xin that it was Tianming who gave her the star, but she just misses him before he dies and his brain is preserved for his spaceflight.
She watches as the Staircase Program is executed, it succeeds in that it achieves 1% the speed of light, but due to a slight error in the direction of the 998th out of 1003 bombs’ explosion, the probe and Tianming’s brain veer off course into the vast expanse of space. With hope lost for the probe and despair for Tianming’s future, she accepts a mission to hibernate and hopes that she can help the future PIA in their future efforts to defeat the Trisolarans.