The sequel to The Three Body Problem has humanity facing some serious problems. A trisolarian invasion fleet is en-route and due to arrive in 400 years, and intelligent subatomic particles called sophons created by the trisolarians mean that strategic deception and advances in physics are impossible.

The Dark Forest follows astronomer and cosmic sociologist Luo Ji, as he attempts to come to terms with what is going on. Luo is the target of repeated assassination attempts by the fifth-column Earth-Trisolar Organization, and then is promoted to Wallfacer, one of four senior strategists with an unlimited remit to create private defense against the alien threat. While the other Wallfacers have plans, as well as ETO operatives to break them, Luo retires to a luxury villa to live a life of peace with his literal dream woman. After all, victory is impossible.

Then his wife and child disappear into long-term hibernation to prompt Luo to action. He comes up with a plan to cast a spell on a star, to reveal it's location to the galaxy, and goes into hibernation as well. He emerges in a utopian future, where after a climate disaster that killed billions, humanity lives a life of plenty in underground cities, while powerful battlecruisers patrol the solar system. It seems like humanity will vanquish the oncoming ETO fleet, when a single trisolarian probe destroys all but a handful of ships. It turns out fundamental advances in physics are desperately needed.

But at the last minute Luo finds out that the spell he cast worked, the star was destroyed. The solution to the Fermi paradox is that the galaxy is a dark forest, full of predatory species that pounce on any species foolish enough to reveal itself in preemptive self-defense. Luo has a plan to reveal the location of Earth and the trisolarian world. The two species would join each other in death. With this mutually assured destruction in place, the trisolarians sue for peace.

There are some parts of the book I thought worked quite well. The build up of the might of Earth's fleet, and its destruction, was quite emotionally effective. The idea that the space force would have political officers to ensure faith in victory across centuries is both sensible, and a distinctly Chinese approach to space navies. Liu Cixin touches on weighty ideas beyond the titular dark forest, including future politics, ecological collapse, underground cities, and limited mind control, but the story has a little more room to breath. Almost too much room to breath, as chapters are wasted on Luo Ji's indulgences. I much preferred Ye Wenjie as a protagonist.