Plot
In a flashback, Chinese sociology professor Luo Ji has a chance meeting with Ye Wenjie, the astrophysicist from The Three Body Problem who first contacted the Trisolarans and encouraged their invasion. She suggests he study “cosmic sociology” based on a set of axioms that she shares with him.
In the present day, the UN is trying to find a way to survive the invasion of the Trisolaran fleet, which will arrive in about 400 years. Their prime difficulty is the sophons, super-smart subatomic machines sent from Trisolaris that see everything on Earth and block advances in fundamental physics, and can communicate with Trisolaris in real time through quantum entanglement. The UN must also deal with an organization of Trisolaran loyalists on Earth who support the invasion and carry out acts of sabotage based on instructions from the sophons.
The sophons can overhear any conversation and intercept any written or digital communication, so the UN initiates the “Wallfacer" Program. Four individuals are granted vast resources and tasked with generating and fulfilling strategies that must never leave their own heads. Three of the Wallfacers are famous for their military, political, and/or scientific expertise; the fourth is the obscure professor Luo Ji, who does not initially understand why he was chosen.
Trisolarans have evolved so that their thoughts transmit openly, and as a result the entire species is unfamiliar with concepts like lies, misdirection and subterfuge. To overcome this weakness, the Trisolarans select three loyalist humans dubbed "Wallbreakers," who are given full access to the information collected by the sophons and tasked with deducing the plans of the Wallfacers. Within a few years, two of the Wallbreakers successfully ruin the strategies of their respective Wallfacers. The third Wallfacer enters hibernation to continue his efforts in a future era, but ultimately his strategy also fails.
Luo doesn’t view himself as capable and refuses the position of Wallfacer, but finds himself unable to do so. All his actions, including announcing his resignation, are interpreted as a ruse in service of his ultimate strategy. Selfishly, he takes advantage of his power in order to secure an isolated paradise and an idealised partner. After several years of peace and happiness, his wife reveals her own plan. She wants him to solve the Trisolaran crisis and so puts herself and their daughter into hibernation until the Trisolaran fleet arrives.
Luo learns that he was chosen as a Wallfacer because he is the only human that the Trisolarans have ever directly targeted for assassination, suggesting that the aliens fear him for some unknown reason. Luo concludes that he must have been targeted because of his conversation with Ye Wenjie, which the sophons presumably overheard. After reflecting on the conversation, he comes to a realization. Luo arranges for the location of a star 49.5 light-years away to be broadcast throughout the galaxy, without explaining his purpose. He then enters hibernation, asking to be woken if an effect on the star is noticed. Soon after the broadcast is sent, the Trisolaran fleet launches a series of probes with faster acceleration that will reach the Solar System much sooner than the rest of the invasion force.
Luo is revived 200 years later in a wealthier, more advanced society and is told that the Wallfacer program has been terminated. Though its technology has been limited by sophon sabotage, Earth has still managed to build a fleet of ships that are larger and faster than the Trisolaran vessels, and world leaders are confident that they can defeat the invasion force. This triumphalism is shattered by the arrival of the Trisolaran's first probe, which proves to be far more technologically sophisticated and annihilates humanity’s combined fleet in minutes. Earth collapses into mass hysteria and the few ships that weren’t destroyed flee the Solar System to form new civilizations.
The UN discovers that the star whose location Luo had broadcast has been eliminated by an unknown hyper-advanced civilization, and his Wallfacer status is quickly restored. However, Luo says his strategy has already been foiled. After destroying the fleet, the Trisolaran probe stations itself near Earth and begins transmitting a steady jamming signal, preventing any further interstellar communication.
Privately, Luo explains the dark forest hypothesis that underpinned his strategy, which he came to understand after reflecting on his conversation with Ye Wenjie: There is life everywhere in the galaxy, but since growth is constant and resources are finite, each galactic civilization is strongly incentivized to destroy any others upon discovery. The only defense against this is to remain unnoticed, thus explaining the Fermi paradox.
Luo had intended to force the Trisolarans into a truce by threatening to broadcast their home planet's location to the galaxy, creating a mutual assured destruction scenario. With the Trisolaran probe thwarting such communication, a forlorn Luo spends his time on a UN project involving distributing nuclear bombs throughout the outer solar system in order to create cosmic dust that will help detect, but not prevent, the arrival of the main Trisolaran fleet.
However, this is a ruse. His true goal is to circumvent the probe's transmission jamming. Luo reveals to the sophons that the bombs have been arranged in such a way that the cosmic dust pattern created by their detonation will reveal to all observers Trisolaris' location. In exchange for leaving the bombs undetonated, Trisolaris accepts the truce, diverts its fleet, and agrees to aid human scientific progress.
Years later, Luo, reunited with his wife and daughter, converses briefly with the pacifist Trisolaran who had made first contact with Ye Wenjie. They agree that it may be possible for galactic civilizations to brighten the “dark forest” and engage in peaceful relations.
