Princess Ella, a mermaid, possesses the mystical power to sense the Black Tide, yet the royal family regards her merely as a tool to guard their secrets. She is deeply in love with her fiancé Cedric and once sacrificed her life to save him. But on the eve of their wedding, she discovers Cedric has been misled by her younger sister Vivian. Not only does he betray her, but Vivian also pushes Ella into the deadly Black Tide. Trapped in mortal peril, Ella is rescued by Ronan, the Black‑Sailed Pirate King. This man, whom she once saved by chance, takes her away from the cold royal palace and grants her freedom for the very first time. As the two venture deep into the Black Tide to uncover the truth, Ella stumbles upon a staggering conspiracy concealed by the mermaid royal family to manipulate the seas. Plagued with regret after losing Ella, Cedric is too late. The mermaid princess who was once sacrificed has fully awakened. She commands the Black Tide, leads the pirate fleet, and exposes the royal family’s lies. From a manipulated pawn, she rises to become the Queen of the Black Tide who holds the ocean’s fate in her hands.