Following the events of
The Three Body Problem, humanity unites to defend Earth against the invasion fleet of the alien planet Trisolaris, which will arrive in approximately 400 years. Earth's defence is severely hampered by Trisolaran "sophons", omnipresent but unobservable supercomputers that spy on all Earth activity and interfere with technological development. However, as sophons cannot surveil human thought, the
United Nations appoints four "Wallfacers": individuals tasked with developing defence strategies known only to themselves, and are provided near-unlimited resources to do so. Three of the Wallfacers are renowned for their military, political and scientific expertise, but their secret strategies are ultimately exposed by Trisolaris' agents. The fourth Wallfacer is the obscure
sociologist and ex-
astronomer Luo Ji, chosen because he is the target of the only assassination order ever issued by Trisolaris. Luo realizes the Trisolarans uniquely fear him because of his prior encounter with astrophysicist
Ye Wenjie, who had encouraged Luo to develop the "
dark forest hypothesis", whose central claim is that a civilization which broadcasts its location to the universe will almost certainly invite its destruction by another civilization. Luo tests his hypothesis by broadcasting the location of a star 50
light-years away, then entering
hibernation while awaiting to see if the target star is destroyed by an unknown third-party civilization. The Trisolarans, realizing Luo has uncovered a way to destroy Trisolaris, send probes to jam any further cosmic broadcasts. 200 years later, Luo is revived into a utopian society boasting advanced technology. However, the widespread belief in Earth's superiority is shattered when its entire space fleet is annihilated by a single Trisolaran probe. Several ships escape the battle into deep space, but they eventually cannibalize each other for scarce resources, with two ships prevailing:
Blue Space and
Bronze Age. Luo considers what happened between the fleeing ships to be a manifestation of the ruthless space sociology predicted by his "dark forest" hypothesis. Amidst the ensuing panic on Earth, Luo resumes his Wallfacer role and successfully develops a system that can circumvent the Trisolaran jamming of cosmic broadcasts. Threatening Trisolaris with
mutual assured destruction should he broadcast Trisolaris' (and by proximity, Earth's) location, Luo forces Trisolaris to divert its invasion fleet and to aid human scientific progress. == Characters ==