The story picks up where The Three-Body Problem left of, though only with a smattering of return characters. The focus is on Luo Ji, a failed academic (specialising in astronomy, moonlighting as a sociologist), who at the beginning of the novel is given the foundational axioms for a sociology of cosmic civilisations.
First: Survival is the primary need of civilisation. Second: Civilisation continuously grows and expands, but the total matter in the universe remains constant.As well as these axioms, there are two other principles at the heart of cosmic sociology: the chain of suspicion, and the technological explosion. How these fit together only becomes obvious at the end of the novel. Luo Ji is given these by Ye Wenjie, who readers will recall brought the doom of Trisolaris upon humanity in the first novel. This is an interesting move as it will eventually be the axioms of cosmic sociology that bring humanity (apparent) salvation at the end of the novel.
Why, though, is cosmic sociology axiomatic? Because of the absence of evidence from which we could build an empirical sociology. Thus, in order to build a sociology of cosmic civilisation, one must reason from acceptable first principles. Luo Ji does precisely this.
I said above that this is hard SF, and it is, but the focus is less on science or technology than one might think. Liu spends much of his time considering strategy. Indeed, the novel feels almost like a puzzle that he has set himself, and the progress of reading it is us following him towards a solution.
What is the puzzle? The puzzle rests on how to fulfil the first axiom, namely, how does one assure the survival of one's civilisation? The following conditions are what Liu sets himself to work with.
- Humanity cannot make any fundamental scientific progress beyond what it had at the beginning of the 21st century.
- Humanity has roughly 400 years to prepare itself for war with a foe who are substantially more technologically and scientifically advanced (including in fundamental science) than humanity.
- The foe can monitor any and all events that take place within the Solar System in real time, but cannot interfere above the atomic level. They can, however, communicate with individuals and groups of individuals.
- The foe cannot read minds. Thoughts remain unknown.
- The foe is ignorant of strategy and deception, as the foe's natural form of communication is such that thought and speech are one-and-the-same.
In order to counter the threat from Trisolaris, and aware of the fact that Trisolaris cannot read minds, the UN establishes the Wallfacer Project. The Wallfacers are four individuals who are entrusted with huge resources, and instructed to use them to save humanity. They are never required to explain or justify themselves, and are encouraged to conceal their true strategy from everyone, both allies and enemies. The reason behind this is that, although humanity cannot hope to defeat Trisolaris technologically, they can hope to exploit their enemies poor comprehension of strategy and deception to outsmart them.
Luo Ji is one of the Wallfacers, the other three are Frederick Tyler, an American military tactician and former Secretary of Defence; Rey Diaz, the former president of Venezuela (a more competent Hugo Chavez who defeated the US when they invaded his country); and Bill Hines, a British neuroscientist.
Trisolaris is deeply concerned about the Wallfacers, and their agents on Earth establish the Wallbreakers, whose task it is to guess the strategy behind the Wallfacers' projects and reveal them to the world. Eventually, all but Luo Ji's strategies are unmasked and their plans fail. Eventually, we learn what Luo Ji's plan is: threatening to reveal the location of the Trisolaran homeworld to the rest of the galaxy.
His strategy is born out entirely from the axioms of cosmic sociology. Assuming the first two principles, and the chain of suspicion and technological explosion, Luo Ji realises that any cosmic civilisation that reveals itself to the rest of the galaxy will be destroyed by any civilisation that learns of its presence, and has the means to destroy it. Why is this? Because each civilisation will pursue its own survival above all else, each civilisation grows but there is only a finite amount of matter in the universe, and any contact with another civilisation will immediately fall into the chain of suspicion. That is, due to the sheer otherness of alien life, and the impossibility of direct and meaningful communication over interstellar scales, it is impossible to determine the motivation another cosmic civilisation will have in alerting others to its presence. Further, the exponential growth of technology (the technological explosion) means that it is possible for a small and primitive civilisation to become advanced and powerful in a (cosmically speaking) short amount of time. As such, it is always more reasonable to assume that attempted contact is a trap, and a trap necessarily implies hostility, and hostile entities must be eliminated (in accordance with the first axiom).
As such, Luo Ji realises that if he reveals the location of Trisolaris to the rest of the galaxy, he will bring inevitable doom upon them (and also the Solar System, as the proximity of the Trisolaris system to our own will almost certainly mean that humanity is detected). This is his grand strategy, and with this he is able to outsmart the invaders.
Like I said, it feels like a puzzle, a challenge Liu set himself to solve over the 550 pages of the novel, and its very rewarding to follow his reasoning. If you're thinking that this makes the novel very cold... you're mostly right. Although the writing is very elegant throughout, characterisation is almost non-existent. We hear only what characters do, and very little about their inner life (which may very well be the point: the novel is itself a Wallfacer).
In the end, The Dark Forest is more like a beautiful piece of clockwork than anything else, both precise and oddly chilling.
Check out this deeply haunting fan film, depicting the 'Droplet' sequence from the novel.