The events of The Dark Forest occur after those in The Three-Body-Problem. Earth is under the surveillance of a hostile alien race known as the Trisolarians that is on their way to invade. Sub-atomic devices known as sophons constantly watch the activities of humans and disrupt any technological advancements. However, these surveillance devices are unable to reading the human mind and the UN begins the Wallfacer program, leaving all their resources in the hands of four men who are entrusted with plans to save the earth from the incoming invasion.
Luo Ji is a Chinese astronomer and sociologist who is one of the four chosen as a Wallfacer. However, he does not want to be a Wallfacer, which leads the UN to kidnap his wife and child to urge him to begin work on repelling the Trisolarian invasion. With little choice, he begins work and develops a method to broadcast specific locations to the entire galaxy. This leads to the Trisolarians taking notice of Luo Ji due to their fears of having their location exposed and they attempt to kill Luo Ji. Trisolarian sympathizers on Earth attempt to infect him with a genetically modified potent disease which almost kills him, leading the UN to cryogenically freeze him until they are able to cure him.
After two decades in hibernation, Luo Ji is awoken to a completely different altered world. The earth's surface has been scorched and become desert-like. Humanity now lives in massive, technologically-advanced caves under the surface of the Earth. Earth has also achieved interstellar travel with a massive fleet of spaceships. The UN's confidence in the space fleet leads them to think that they can repel the incoming Trisolarian invasion and to disband the Wallfacer project.
However, when a Trisolarian probe arrives and confronts the entire fleet of Earth's spaceships, it is able to single-handedly destroy almost all of the fleet. During this time, Luo Ji and the Wallfacer Head of Security, Shi Qiang, are able to verify the effectiveness of the method of broadcasting locations he discovered 200 years prior. Armed with this knowledge, Luo Ji concludes that the Fermi Paradox would be proven because every civilization would only think for its own survival and the finite nature of resources create a situation where it would be best for each civilization to wipe out every other.
Luo Ji then threatens to kill himself and trigger a mechanism that will broadcast the exact location of Trisolaris to the galaxy. This leads the Trisolarians to concede and agree to remove the sophons from earth and help humanity advance its technology.
