Graduate student Isla studies merfolk on planet Thalassa. She mistakes ancient siren Opal for a deep‑sea merman and teaches him courtship. Bound by forgotten childhood bonds, Opal abducts her, believing she is his fated mate. Trapped in a sea cave, Isla recovers lost memories. After trust fractures and a pregnancy crisis, she sets Opal free. He chooses to stay, and they embrace their future with their unborn child.
