This is the second book in a trilogy that starts with The Three-Body Problem.
As The Dark Forest opens a large fleet of hostile Trisolaran aliens is headed for Earth, destined to arrive in 400 years. Presumably this is when the 'Doomsday Battle' will occur between the human race and Trisolarans.
Humans are aware of the danger but are hampered in making preparations because the aliens have sent 'sophons' to spy on Earth. Sophons are souped up protons that (due to quantum physics) can INSTANTANEOUSLY transmit every single thing humans say and do to the Trisolarans. Thus, the aliens can 'hear' all discussions about how to combat them and 'see' all weapons being developed. To make matters even worse, the sophons have blocked technological advancements, curtailing Earth's ability to develop the most effective/advanced weapons systems.
The Trisolarans one weakness is their inability to read human minds. Thus, the United Nations institutes the 'Wallfacer Project' in which four individuals - called Wallfacers - are instructed to develop a strategy to fight the Trisolarans. Each Wallfacer is to work alone, write nothing down, and use evasive measures to flummox the Trisolarans. The Wallfacers have almost unlimited resources with very little accountability, so they can do pretty much whatever they want. And one Wallfacer seems more concerned with having a comfortable life than with fighting the Trisolarans.
The Trisolarans, in turn, designate four humans sympathetic to their cause to be "Wallbreakers" - whose mission is to sabotage the Wallfacers' plans. The Wallfacers are very clever and inventive, but the Wallbreakers are pretty smart too. (The author has clearly done a lot of research for these books.)
Around the time this is going on some humans - including people with serious illnesses, a couple of Wallfacers, and various professionals - are put into hibernation. The plan is to awaken them at a later time when they can be cured and/or be useful to humanity.
Skip ahead two hundred years. Some hibernators have been revived and Earth looks very different. There are well-designed underground cities as well as large fleets in space, which are now considered to be separate 'countries.' The Trisolaran fleet is due in two centuries, but the aliens have launched a fast 'probe' which will arrive any day. Oddly enough, people seem to be relatively optimistic. Some think humanity will win the Doomsday Battle while others believe the Trisolarans might turn out to be friendly. Surprises abound after the probe enters the Solar System, and things take a rather dramatic turn...all very exciting.
There's a good deal of philosophical underpinning to some of the plot developments. For example, 'escapism' - the plan to launch some people into space to preserve the human race (just in case) - is outlawed, presumably because there's no fair way to decide who will go. Is this right?
And when there are limited resources and too many individuals, what should be done? And IF the Trisolarans are defeated, should alien survivors be treated in a 'humane' fashion? (This debate reminded me of American Indian history as well as the movie 'District 9.') All things to think about.
I found the story a bit dense and slow-moving but overall I enjoyed the book, which is full of inventive ideas and interesting characters. I don't think it's giving away too much to say that - at the end of book 2 - there are still humans and Trisolarans. I'm interested to see what happens in the final volume of the trilogy.
Overall, I'd highly recommend this series to science fiction fans.