Mermaid Irene saves Cassian, a chaebol heir left for dead in the deep sea. Falling in love, she trades her voice and inherited conch shell with a sea witch for human legs. However, a wicked woman named Vera steals the conch and claims to be Cassian's savior. Infiltrating his mansion as a lowly servant, Irene suffers brutal abuse from Vera, who forces her to dance on broken glass. When her mermaid identity is exposed, a brainwashed Cassian smashes her knees with a hammer to harvest her scale bones, throwing her out like garbage.
Irene crawls back to the ocean, where Leon, the crown prince of the deep sea, rescues her and destroys Cassian's empire. When the truth is revealed through the conch shell, a devastated Cassian smashes his own knees and grovels on sharp coastal rocks for days, but Irene rejects him with utter disgust. To atone, Cassian later takes down a gang of mermaid hunters at the cost of his life, leaving his wealth to protect her ocean. Reborn and healed, Irene leaves his ghost behind, stepping into the endless brightness of the deep sea side by side with Leon.
