Although *The Three-Body Problem: Earth’s Past* features two major climaxes—the "Guzheng Operation" and the "appearance of the Sophons"—most of its content serves as setup.
It establishes the key characters like Ye Wenjie and Shi Qiang, the ETO organization, and the core conflict of the entire book: the alien civilization’s invasion of Earth.
In writing, this is called "setup." But in other contexts, this is called... foreplay!
Chapter 422