Cassarah returns to Sirenia's Haven, a mermaid haven, to study its ecosystem. She is drawn to Imogene, a mermaid, but their connection is challenged by their separate worlds. Meanwhile, Cassarah's colleague Priya discovers a hidden pact between humans and Merfolk. Together, they fight to protect the haven from a corporation that threatens its secrets. As they win the battle, Cassarah must choose between Imogene and Priya, and a new alliance forms between humans and Merfolk.