After the sea level swallowed up the land, humans established hierarchical city-states in the high-pressure deep sea, and oxygen became currency and shackles. Lan discovered that the "perpetual tidal engine" that the city-state relied on for survival was actually a self-destruction device that extracted energy from the earth's core. When she tried to reveal the truth, she was framed as a traitor. She could only take her young brother to escape to the legendary "Old World" that had been submerged in the sea. During the suffocation and pursuit, she looked for the possibility of humans returning to the surface.
