In the years Cheng Xin hibernates, humanity builds a massive space fleet to protect itself from the Trisolarans. In the meantime, the Trisolarans send several strong-interaction material droplets to the Solar System—small unmanned spaceships that are almost indestructible. At first, the humans believe that the droplets are a gift of peace, the promise of shared technology. However, the droplets soon attack the new human space fleet, which cannot defend against them. In this Doomsday Battle, the droplets obliterate almost every ship, though two small groups of ships manage to flee into deep space.

Each escaping group of ships is soon reduced to one, Bronze Age and Blue Space, whose crews destroy the other ships, stealing their resources to better their chances of survival. Fourteen years later, Bronze Age decides to return to Earth.

Of the many Wallfacer plans, only Luo Ji’s is a success. At first, what he does is mystifying: He broadcasts the location of Star 187J3X1—a star unconnected to Earth or Trisolaris—to the universe. However, soon, that star’s system is destroyed when an object is launched into its sun at near light-speed.

Luo Ji has realized that the universe is like a dark forest, in which civilizations seek to destroy any others they discover to protect themselves and gain access to finite resources.