Cheng Xin's time as Swordholder comes to a quick end as 6 Trisolaran probes speed their way to Earth. She has only 10 minutes to consider whether or not to activate the deterrence system. She realizes that she cannot erase the 3.5 billion year history of the Earth and throws away the button. Shortly after, the probes reach Earth and destroy all of the gravitational wave antennas on Earth, disable the dark forest deterrence, but leave Cheng Xin unharmed.
When Cheng Xin and AA go to examine the impact site, she is met by Sophon who tells her that Trisolaris predicted this outcome for years, Gravity has also been destroyed, and finally says “Get ready to go to Australia, you pitiful bugs.”
Over the course of the next year, the entirety of humanity is forced to relocate to Australia. Conditions quickly devolve into a fight for food and resources, even with humanity even turning to theocracy and dictatorship. An Earth Security Force is established to force people who are not willing to relocate, eventually turning to indiscriminate killing of people.The Earth Resistance Movement is also formed to fight the Trisolarans, despite the fact that all of their moves can be tracked by the Sophons.
Sophon announces their ultimate plan, to defang and force humanity to fight over resources to the point where they’re forced to eat each other and get the human population to about 50 million people. Upon hearing this, Cheng Xin finds herself going blind.
Aboard Gravity, James Hunter, who has been entrusted with destroying the gravitational wave antenna should anything happen, but once the droplet attack starts to happen, he finds that the system to destroy the antenna is not functioning. Soon after, the droplets change course and seem to die, and finally they’re boarded by the Blue Space.
The crew of the Blue Space lays out what they believe has happened: The Trisolarans had attacked Earth and coordinated the droplet attack to coincide with the Swordholder handover ceremony. After a long discussion, they vote as a combined crew to activate the universal broadcast system and send the gravitational waves into the dark forest of space.
Blue Space then reveals the secrets of how they disabled the droplets and what the strange phenomena they have been seeing. In this region of space, they had been encountering fourth dimensional “fragments”, which they are able to enter and experience interacting with the world in higher dimensions.
During further exploration of 4D space, they encounter beings that reside there. They are able to find a way to communicate with a ring shaped object, and it cryptically tells them that it is a tomb, and that they are fish in a puddle of a drying sea. After exiting the 4D space, they observe the fragments shrinking, and finally the ring itself losing its fourth dimension.
Meanwhile back on Earth, a year after the Great Resettlement had been completed, the news that the universal broadcast was received by Trisolaris was met with cheering from humanity and a retreat from the Trisolarans. Humanity would be returning home from Australia.