<Arrrg, matey, spoilers ahead! Ye have been warned!>
A young astrophysicist, Ye Wenjie, is working at a secret SETI base in the late 1960’s/early 1970’s. Her professor father was disgraced, beaten, and killed by Red Guards in the Cultural Revolution, and Wenjie had been working at a nearby logging camp before being pulled into work more suited to her talents. One day she stumbles upon a method of using the Sun as an amplifier for the radio greeting they are sending out into the cosmos. She thinks it is a failure. But several years later, a return message comes. It’s actually a dire warning – “do not answer!!!” An advanced, militarized race detected the first transmission and any others will give them a firm fix on Earth. But, Wenjie is still pretty ticked off about her father and the whole Cultural Revolution in general (who wouldn’t be?) … and replies: “Come here! Our civilization is no longer capable of solving its own problems.”
And thus a mega force of “Trisolarian” invaders is on the way, ETA around 400 years (they travel pretty fast, but still much slower than radio transmissions). Also, they have “folded” (?) protons into super-AI Sophons which are already at the Earth, messing up physicists’ particle accelerator experiments. The Trisolarians were worried that human technology, while currently inferior to their own, was progressing just too exponentially to leave alone for 400 years.
The Trisolarians are named such because they live on a planet revolving around three stars (Alpha Centauri) which experiences an unstable mix of super hot and super cold periods depending on relative distance to each star. Civilization is routinely (yet randomly!) destroyed in either fire or ice. After millenia of trying to figure out what was going on with their world, then trying unsuccessfully to solve the three-body problem, they ultimately determine to find some better planet and move. Wenjie’s description of Earth sounds nice…
Wenjie finds plenty of sympathizers on Earth who agree that humanity needs help, or even that it deserves to be eradicated. I thought this was going a bit too far – are there really that many eco-terrorist types out there who would root for the aliens over humanity, including their own self and family? Especially the character of Michael Evans in the book was really hokey. Billionaire tree-hugger who wants all humans to die and leave the birds and bugs alone.
Some of the group more-or-less worship the unseen Trisolarians (some interesting commentary in the book on how even a solitary confirmation that ET exists, and nothing more, would still fundamentally alter civilization) and put together an odd MMORPG, 3body, to tell about the Trisolarian world and history. It’s through this game that the protagonist (and we the reader) first learn about what’s going on. Little bit of reveal at a time.
There is a REALLY funny incident in the game, where a 30 million man medieval Chinese army becomes a von Neumann architecture computer: squads of soldiers with black and white flags become logic gates, scribes become memory, and cavalry becomes the bus. Pretty ridiculous but funny.