When a fall from the mast leaves Alba Marsh unable to sail, he finally has the chance to run away from his family debts in search of a better life. He hopes to reunite with his mother, Edythe, where they always promised to meet if things went awry, but finding the small, secluded, secretive town of Moon Harbor proves a greater challenge than Alba ever expected. Worse, upon arriving, no one seems to know anyone by the name Edythe Marsh at all.

Taking a job as the local lighthouse keeper while waiting for her to come, Alba attempts to find peace in his stolen freedom--but the sea has other plans. First come the shadows, then the voices, then watery trails left on the floor while he sleeps. He's inundated with erotic dreams of sirens calling out to him, and distant singing haunts his waking hours, all while the townspeople claim to hear nothing.

After a debtor catches up with him and he's forced to dispose of their body in the harbor, Alba is interrupted by Eridanys, the last merrow of Moon Harbor, who curses him from leaving until he can find out what happened to the other merrow that once filled those waters. Eridanys' demands, both mental and physical, are exhausting and endless--but as their relationship deepens, and Moon Harbor's long-kept secrets are butterflied open to see, it becomes clear they may both be prey to the town in ways neither of them ever anticipated.