In a siren-infested sea, Thomas is the last survivor on a sinking ship, the only sailor unaffected by the sirens' song. An ancient ocean god becomes his unlikely savior when she offers to rescue him in exchange for his help. She believes Thomas is the key to finding a cure for the plague that has been turning mermaids into sirens around the world.

As Thomas embarks on his journey, he is thrown into the unfamiliar world of magic and discovers it is nothing like he imagined. The memory of his forbidden first love haunts him, and the infamy of his name follows him. His comfortable reality slowly crumbles as he learns his family has been hiding a terrible secret.

As he grapples with his new place in the world, he uncovers a powerful force causing the mermaids' illness. If he can't stop it from spreading, merfolk and humankind alike could be in far more danger than he realized.