Elias, a marine rescue worker, saves an injured merman prince named Caelan during a storm. Caelan is a royal heir fleeing a coup, and Elias discovers he carries the blood of a "Keeper" — a human who can open the gate to the underwater kingdom Thalassar. As they hide from human hunters and deep‑sea assassins, their bond intensifies into a forbidden, obsessive love. Elias learns that his brother Jonah's drowning was not his fault but part of a conspiracy led by the scientist Evelyn and the usurper Nerion. Together, Elias and Caelan forge a mystical Tidebound pact, confront the usurper, and seal the dark rift. Caelan rejects the throne, choosing to build a council instead, and returns to Elias to co‑found a marine sanctuary. But in the abyss, a black crown is rising, and a new threat whispers that the deep will choose another ruler.
