Death's End

The story begins during the Fall of Constantinople, describing a prostitute who gains the power of retrieving objects and cutting out human organs without penetrating their enclosures. Constantine XI tasks her with killing Mehmed II, but her powers are mysteriously lost, and she is killed by Byzantine soldiers in retaliation.

Next, the story shifts to the timeframe of the beginning of The Three Body Problem. The physicist Yang Dong, daughter of Ye Wenjie, having discovered her mother's conspiracy with Trisolaris, and witnessing the disruption of the particle-accelerators, is eventually driven to suicide. Prior to her death, she meets another person at the lab (later revealed to be Ding Yi) who insists that life and geography evolved together, as opposed to the latter merely having enabled the former.

Now, the timeframe is the beginning of The Dark Forest. Around the time that Luo Ji is appointed as a Wallfacer, an aeronautical engineer named Cheng Xin is working for the Staircase Project, which aims to launch a human toward the Trisolaran fleet at 1% of light-speed, someone who can serve as an informal diplomat for earth while gathering intelligence. The seemingly impossible speed-goal is realized through Cheng's idea of lining up ICBMs to create a nuclear catapult. However, the mass of the vehicle is severely constrained, and Wade, the CIA agent leading the project, decides to find some person to euthanize, and to send only the brain, on the assumption that the Trisolarans will be able to reconstruct the body.

Meanwhile, a terminally ill engineer named Yun Tianming recalls his years-long affection for Cheng. Upon receiving an unexpected sum of money, he buys the title-deed to the distant star DX3906 from the United Nations, which he anonymously bestows upon Cheng. A few days later, Cheng sees Yun's name on a list of terminally ill persons, and visits him for the first time in years, persuading him to volunteer for the Staircase Project. Upon learning that Yun was the donor of the star, she feels overwhelmed with guilt. When Yun's brain is launched into space, a malfunction causes his spacecraft to go off course into deep space, and he is thought to be lost forever. Afterward, she accepts an opportunity to hibernate, in order to serve as the Staircase Project's liaison for future generations.

At the end of The Dark Forest, the Trisolaran invasion was averted owing to Luo's threat of Mutually Assured Destruction (MAD), which involves broadcasting the position of the planet Trisolaris across the universe, thereby attracting the attention of hostile alien races. In the event of such a broadcast, Earth would be exposed as well due to its proximity to Trisolaris. Luo is appointed to be the first Swordholder, the person responsible for launching the broadcast in the event of any further Trisolaran aggression.

With safety restored, the defecting ships Bronze Age and Blue Space are seemingly invited to return. However, upon the return of Bronze Age, its crew is imprisoned for crimes against humanity. The leader of Bronze Age warns Blue Space not to return, so the human ship Gravity and two Trisolaran droplets join forces to chase after Blue Space.

Fifty years pass. Cheng wakes up from hibernation, starts a company with an astronomer named AA, and becomes the leading candidate for the next Swordholder. Wade desires the position and tries to murder Cheng, but is arrested and thrown in prison. Right after Cheng's inauguration, the Earth is attacked by Trisolaran droplets, and humanity's gravity wave-transmitters are destroyed, with the Trisolarans correctly guessing that Cheng would not send the MAD broadcast. At the same moment, the two droplets working with Gravity attempt to destroy Gravity and Blue Space. However, the crew-members of Blue Space have discovered a fading "four-dimensional fragment" in space, which they exploit in order to destroy the two droplets and capture Gravity. The crew-members of the two ships make peace, and decide to send out the MAD broadcast using Gravity's antenna. Afterward, Gravity joins Blue Space in escaping the solar system.

The Trisolarans begin massacring humans using the droplets, and have forced the entire human population to move to Australia, where they are to be systematically starved to reduce their numbers to manageable levels. Upon detecting the MAD broadcast, the Trisolarans abandon their invasion of the Earth. A few years later, as humanity recovers, one of Trisolaris's three suns is struck by a relativistic "photoid" launched by unspecified aliens, leading to the planet's utter incineration. It is understood that sooner or later, the Solar System will suffer a similar attack.

The remnant Trisolarans reveal that they have Yun in their custody, and allow a single, heavily supervised tele-conference between Yun and Cheng. Yun, having been treated by the Trisolarans as an honored guest, and granted access to the Trisolaran data-banks, delivers a complex fairy-tale which contains three cosmological secrets, two of them being subsequently deciphered by the human council: that light-speed travel can be attained by warping space through "curvature propulsion", and that it is possible for a civilisation to reduce the speed of light so that they cannot escape their own star system, thereby forming a "black domain" which no one will attack as its inhabitants cannot affect the space outside.

Humanity decides to heavily invest in space-cities possessing artificial light-sources, which can hide in the shadow of Jupiter to escape any photoid-attack against the sun. As time goes on, nearly the entire Earth is evacuated. Research into curvature-propulsion is banned owing to resentment from the populace, who regard the technology as an escape-hatch for the ultra-wealthy, and owing to the discovery that such propulsion creates permanent rifts in space-time. Cheng strongly disagrees with this policy, and accepts a request by Wade, now out of prison, to transfer all of her private wealth to him, so that he may assemble a secret research team.

Cheng hibernates for sixty years, and is woken at Wade's request after the latter's adherents get into an armed stand-off with the government. Worried over the danger to civilians caused by Wade's new anti-matter bullets, Cheng demands Wade's surrender, reminding him that she has final say in the venture, and Wade reluctantly orders his security-forces to stand down. Contrary to expectations, the government shows no mercy to Wade on account of either his voluntary surrender, or the quality of his research. Wade is executed, to Cheng's dismay, and Cheng, feeling that her life has no direction, hibernates for another sixty years with by AA before waking up to the news that the alien strike has arrived.

However, the space-cities are totally useless: the attack has come in the form of an object which collapses 3D space into two dimensions. The existence of such weapons was the third secret in Yun's story which the interpreters had neglected. The only way to survive is either to flee at light-speed, or to have re-engineered one's species to exist in the 2D plane. The entire solar system is flattened and killed, with all ships being sucked in, but Cheng and AA escape using a curvature propulsion ship built in secret by Wade's associates after his death.

Cheng and AA travel at light-speed to Planet Blue, a planet orbiting DX3906. The journey takes 287 years, but from their perspective, only two days. There, they encounter Guan Yifan, a crew member of Gravity, who explains that the crew of his ship and Blue Space went on to develop curvature-propulsion and colonize four planets (in different systems). However, one of those planets became fearful of detection, and placed itself within a Black Domain.

Cheng and Guan fly to the nearby Planet Gray to investigate signs of alien activity, whereupon they discover Death Lines (similar to black holes) which were laid down by a faction of aliens purposely trying to accelerate the ruin of the universe. At this point, Guan explains the larger picture to Cheng: the universe is slowly being torn apart by galactic warfare. In old times, the universe existed in 10D, and the speed of light was faster, but as time went on, galactic civilizations continually re-engineered themselves to occupy one fewer dimension, and then tore apart the topmost dimension as a way of killing their enemies. Likewise, some civilizations created black holes to serve as shields against foreign attacks, but this led to the reduction of light-speed, owing to the disruption of space-time.

As Cheng and Guan fly back to Planet Blue, they are notified by AA that Yun has arrived. Cheng is overjoyed that they will finally reunite. However, the Death Line suddenly expands and traps their ship within it for sixteen days. When they are released, they discover that 18 million years have passed in the external world, and that the speed of light has been reduced by a factor of 10,000, rendering most electronics useless. They find a message revealing that AA and Yun lived a happy life together, and prior to dying left behind a gift for Cheng and Guan: a parallel universe measuring one cubic kilometer, made with Trisolaran technology, containing an idyllic farmstead to which Cheng and Guan can retire, while they wait for the main universe to die and be reborn as the Garden of Eden.

After living there for some time, Cheng and Guan receive an alien message aimed at all denizens of micro-universes, stating that the presence of micro-universes deprives the main universe of mass, disrupting its possibility of eternal cycles of expansion, collapse and rebirth. Cheng, being accompanied by Guan, and wearily reflecting on her lifetime of moral duty, disassembles the objects of the micro-universe, and steps back with them into the dying main universe, leaving behind a message in a bottle and also a fishbowl for the reborn universe to uncover.