Content Warning: The source text and this guide contain material related to suicide and violent death.

The Prologue begins with a flashback in the mind of Luo Ji. In a graveyard, an ant crosses a gravestone, exploring the numbers carved into it. On its journey, it passes a spider, its natural enemy but, to its relief, they ignore each other. The ant is present for the arrival of Luo Ji, who is visiting the burial site of his high school friend Yang Dong. Before long, Yang Dong’s mother, Ye Wenjie, joins him and tells him of how Yang Dong used to mention him. Over the course of their discussion, Ye Wenjie suggests to Luo Ji that he take up the study of “cosmic sociology,” a theoretical study of civilizations in the universe and how they interact with each other.

Elsewhere, Mike Evans, one of the leaders of the Earth-Trisolaris Organization (ETO), speaks with Trisolaris through sophons, subatomic particles that function as supercomputers and connect the two planets in real time. The Trisolaran he speaks with expresses confusion over the terms “think” and “say,” as Trisolarans communicate telepathically, making it impossible for them to keep their thoughts and speech separate.