Death's End

During the Fall of Constantinople in AD 1453, a prostitute gains the ability to retrieve small objects in seemingly impossible ways, up to and including the vital organs of heavily guarded human targets. Constantine XI tasks her with killing Mehmed II, but her powers mysteriously vanish, and she is executed. These events are attributed to the passage of a "four-dimensional fragment" through the solar system.

An aeronautical engineer named Cheng Xin is recruited by the Planetary Intelligence Agency (PIA) to work on the Staircase Project, an attempt to launch a probe toward the Trisolaran fleet to gather intelligence. The seemingly impossible goal of accelerating the probe to 1% of lightspeed is realized through her idea of lining up ICBMs, Topol and Dongfeng missiles to launch the probe using nuclear pulse propulsion. The Planetary Defence Council (PDC) at first rejects the plan because of the high intercept speed and the unlikelihood of being able to surveil Trisolaran communications. Thomas Wade, the CIA agent leading the project, proposes making the probe desirable to Trisolaris by putting a live human on board, who would then act as a double agent after being integrated into the fleet. However, the mass of the vehicle is limited to only a few kilograms, so Wade decides to find a volunteer to be euthanized and send only their brain, on the assumption that the Trisolarans will be willing and able to reconstruct their body.

One of Cheng Xin's acquaintances, an engineer named Yun Tianming who has always held a secret affection for her, discovers that he is terminally ill. Upon receiving an unexpected sum of money, he buys the title deed to the distant star DX3906 from the United Nations and anonymously gifts it to Cheng. When Cheng learns of Yun's illness, she persuades him to volunteer for the Staircase Project; he is selected as the subject and his brain is extracted and launched. A malfunction causes the spacecraft to go off course into deep space and the project is written off as a failure. After learning that Yun was the source of the star title, Cheng enters hibernation, in order to serve as the Staircase Project's liaison for future generations.

The Trisolaran invasion has been forestalled by Luo Ji's creation of Dark Forest Deterrence, which threatens Mutually Assured Destruction (MAD) by broadcasting Trisolaris's location across the universe to bring it to the attention of hostile alien civilizations; doing so would likely allow the hostile civilization to deduce Earth's location as well, due to the proximity of the two systems. The deterrence is enforced by a system of gravitational-wave transmitters distributed across Earth and mounted on some spacecraft. Due to concerns about the system being vulnerable to extremist attacks if control is too widely distributed, the sole responsibility for activating the system in the event of Trisolaran aggression is placed in the hands of one person designated the "Swordholder." Luo Ji is the first holder of the position.

Two groups of human starships fled the solar system in opposite directions after the disastrous battle with the Trisolaran "droplet" probe in the previous book, but each group was quickly reduced to one ship after the crews concluded that they could maximize their chances of survival by killing the other crews and harvesting their ships' resources.

The two surviving ships, Bronze Age and Blue Space, are informed of the truce with Trisolaris and invited to return to Earth. Bronze Age arrives first, but the UN puts its crew on trial for crimes against humanity. Its commander manages to warn Blue Space not to return before his execution; the stellar warship Gravity, which is equipped with a deterrent transmitter, sets off in pursuit, accompanied by two Trisolaran droplet probes, although it will take several decades to catch up.

Fifty years pass. Cheng is woken from hibernation due to her possession of DX3906, which has been discovered to have planets by an astronomer named AA. Cheng keeps the title for the star itself but agrees to sell the rights to the planets, and she and AA start an aerospace company together.

Luo Ji is preparing to retire as Swordholder and the UN is seeking replacements; Cheng becomes the leading candidate. Wade, who has also hibernated, desires the position and attempts to murder her, but fails and is given a long prison sentence. Cheng is selected to succeed Luo, but the moment the handover occurs, Trisolaris begins an attack against Earth using multiple droplets to target the gravitational wave transmitters. They have correctly predicted that Cheng won't be able to bring herself to actually carry out the threat and doom two civilizations.

With the deterrence system neutralized, the Trisolarans immediately break the truce, although their ambassador Sophon announces that the aliens have developed an appreciation for human culture and now intend to let a portion of humanity live on, albeit without any modern technology. The droplets begin directly attacking major cities, and Sophon orders all of humanity to relocate to a reservation in Australia, where they are forced to live under appalling conditions in order to rapidly shrink the population before the Trisolaran fleet arrives to occupy the rest of Earth.

In deep space, Gravity catches up to Blue Space at the same moment that the Swordholder handoff occurs back on Earth. Without warning, the two escort droplets move to destroy both ships in order to take out the last remaining gravitational wave transmitter on Gravity. However, the crew of Blue Space has discovered that the ships have entered another four-dimensional fragment; they learn to navigate the fourth dimension and use that ability to disable the two droplets and capture Gravity at the last moment. They use Gravity's transmitter to send the MAD broadcast, then resolve not to return to the solar system. Upon detecting the broadcast, the Trisolarans abandon their invasion and divert their fleet away from the solar system, concluding that Earth and Trisolaris are now both doomed.

One of Trisolaris's three suns is struck by a relativistic "photoid" launched by unspecified aliens, destroying both it and the Trisolaran homeworld. It is understood that sooner or later, the Solar System will suffer a similar attack.

The Trisolarans reveal that they have a fully reconstituted Yun Tianming in their custody, and allow him and Cheng to meet over a sophon-mediated audiovisual link. The conversation is closely monitored to avoid intelligence disclosures, so Yun tells Cheng three elaborate fairy tales, carefully composed to communicate key scientific concepts through metaphor and subtext. Yun invites Cheng to meet him on one of the planets orbiting her star before the link is broken.

The first fairy tale is never decoded even after great effort, and the UN eventually gives up the attempt. The second tale explains that it is possible to reduce the speed of light in a region of space. By lowering the speed of light throughout the solar system to a level below the escape velocity of the sun's gravity, humanity could make the whole system disappear to the galaxy at large, signaling that Earth is harmless and needs not be destroyed, though it would mean trapping human civilization inside the "black domain" forever. The third tale explains that lightspeed travel may be accomplished by manipulating the local curvature of spacetime, using the analogy of a paper boat with a lump of soap attached at one end that will decrease the surface tension of the water behind the boat, propelling it forward.

Humanity decides to construct space colonies in the shadow of the solar system's gas giants, where they could survive the destruction of the sun by photoid attack. Virtually everyone evacuates from the Earth. Lightspeed travel research is banned after it comes to be seen by the population as an escape hatch for the ultra-wealthy, who could use relativistic time dilation to cheat death; it is also discovered that using it leaves permanent traces on spacetime, which could draw further hostile attention to Earth. Wade, who by now has been released from prison, asks Cheng to transfer her considerable personal wealth to him to finance a secret lightspeed research project; she assents after extracting a promise that he will return the wealth and shut down the program at her request.

Cheng hibernates for sixty years, and is woken when Wade's program is discovered by the government. The technology works and provides very promising research in other fields, but Wade has also created a private army equipped with potentially devastating antimatter weaponry. Horrified by this, Cheng invokes the promise, and Wade surrenders. However, the government shows no mercy to Wade, and executes him, prompting Cheng to return to hibernate with AA until the alien strike comes.

Several decades later, a ship from an unknown hostile alien race detects the signal revealing the location of Trisolaris. An operator on the ship investigates and discovers that Trisolaris has already been eliminated by another ship or hostile race, but the operator then reviews the record of radio broadcasts from Trisolaris and deduces that there must also be intelligent life on Earth. Recognizing that the solar system's gas giant planets create "blind spots" that could allow some life to survive a photoid attack, the operator instead launches a more complex weapon at the system.

Humans detect the weapon as it approaches the solar system and send a ship to investigate, but it is quickly destroyed when the weapon is revealed to be a spacetime anomaly that causes the third dimension to collapse into the second; humanity has no defense against this and will be rendered extinct within days as the effect spreads across the solar system. The existence of such weapons was the secret of Yun's first fairy tale; it is later explained that using such a weapon was a gesture of respect by the attacker as they considered humanity a powerful and dangerous enemy. The only way to survive is either to flee at light speed, or find a way to live in the 2D plane; neither appears a realistic possiblity.

One of Wade's former associates awakens Cheng and AA and urges them to fly on one of their corporation's private spacecraft to Pluto, where they meet the now 200-year-old Luo Ji, who gives them a number of artifacts from the Museum of Humanity. They watch from Pluto as the sun and the rest of the planets are pulled into the second dimension and flattened. As the collapse approaches Pluto, Luo Ji then reveals that their ship is equipped with humanity's only lightspeed drive, built in secret by Wade's associates after his death. He instructs them to escape the solar system while he stays behind. They set a course for DX3906 and escape ahead of the collapse wave, which pulls in Pluto and all the remaining human ships that were attempting to flee at sublight speeds, ending human civilization.

Cheng Xin and AA arrive at DX3906 286 years later, though the voyage passes quickly for them due to lightspeed time dilation. They discover that the system has two planets, which they name Planet Blue and Planet Gray. There, they encounter Guan Yifan, a civilian cosmologist of Gravity, who explains that the crew of his ship and Blue Space went on to develop curvature propulsion and founded an interstellar society.

Cheng Xin and Guan Yifan fly to the nearby Planet Gray to investigate signs of alien activity. They discover a series of Death Lines, the seed elements of a black domain, which have been laid down by the aliens in order to hasten the end of the universe. Guan Yifan explains that endless interstellar warfare using curvature- and dimension-based weaponry is degrading the entire universe. In its youth, the universe possessed ten dimensions, and the speed of light was near infinity, but as time went on, galactic civilizations in conflict would collapse regions of space into lower dimensions, forcing each other to permanently migrate there while the higher dimensions ceased to exist entirely. At the same time, the widespread use of black domains resulted in the speed of light permanently lowering throughout the universe, until it degenerated to three dimensions and the current value of c.

As Cheng Xin and Guan Yifan fly back to Planet Blue, they are notified by AA that Yun Tianming has arrived. Cheng is overjoyed that they will finally reunite. However, the activity of Yun's curvature drive triggers the death lines, converting the system into a black domain with a lightspeed only marginally above their current orbital velocity, meaning they are now orbiting Planet Blue at relativistic speed. Guan's spacecraft is equipped with backup systems that are designed to function in a black domain environment, but it will take many hours for them to boot up, forcing Cheng and Guan to hibernate to conserve oxygen. Once the process completes, they realize that 18 million years have passed in an external frame of reference. Upon descending to Planet Blue, they find a message revealing that AA and Yun Tianming lived a happy life together, and have left Cheng and Guan a gift: a portal to a pocket universe, made with Trisolaran technology, containing one cubic kilometer of idyllic farmstead. They enter the universe, meet with Sophon, who was already living there, and wait outside of time for the main universe to die and be reborn in a Big Bounce.

After living there for some time, Cheng Xin and Guan Yifan receive an alien message aimed at all denizens of micro-universes in all languages including Earth's and Trisolaris', stating that the presence of micro-universes deprives the main universe of mass, disrupting its cycle of expansion, collapse and rebirth. Cheng Xin, accompanied by Yifan, wearily reflects on her lifetime of moral duty. They decide to disassemble the contents of the micro-universe and return the mass to the main universe, then join it in death and rebirth. They leave behind a message in a bottle and a self-sustaining terrarium for the denizens of the reborn universe to uncover.