Hai Mo, a mermaid of an unusual species, suddenly discovers that she is living inside a novel—and that she is the sworn enemy of its heroine, Xia Yu. Determined to rewrite her tragic fate in the original story, she conceals her mermaid identity and approaches Xia Yu, becoming her assistant. From mutual suspicion and testing each other's intentions to gradually building trust, the two investigate the successive strange transformations occurring in Wildfire City, fight mutated parasitic creatures, and unravel the villains' conspiracy. Facing the desperate conditions of the apocalypse, they support each other, stand side by side in search of the truth about their origins, and fight together for a chance to survive.