Ballet genius Cecilia is an envied heiress, yet she is exposed as an adopted daughter in public by her rival, betrayed by her fiancé, and surrounded by rumors. Mysterious transfer student Karen appears every time she is in danger, hiding an unspeakable secret in his ice-blue pupils. He is the orthodox crown prince of the deep-sea merman clan, under strict rules never to associate deeply with humans, yet he breaks the rules repeatedly for her. When the regent's conspiracy drags both clans into war, she holds a century-old contract and dives side-by-side with him into the deep sea, shattering racial prejudice with love, jointly initiating the purification array, and finally becoming lovers sharing the tides.