Mermaid Princess Maris, after her sister Cora is captured by a human fishing trawler, endures the agony of scales peeling off to transform her tail into human legs, infiltrating Ashford Estate as a maid. The eldest son, Dorian, carries his own blood vendetta—his stepmother Seraphina pushed his mother Elena off a balcony to seize the family fortune, and his father Magnus knew but chose to cover it up. The two form an alliance through mutual suspicion: Maris aims to rescue her imprisoned sister, who is being drained for her blood, while Dorian seeks to avenge his mother.
They secretly record the stepmother's conspiracies, uncover Elena's diary and final words, nearly die in a stairwell ambush, and finally destroy the antagonists after a paternity test reveals Caspian is not Magnus's biological son. On the rescue night, Maris smashes the holding tank to free the dying Cora, while Magnus collapses and dies from serum withdrawal during the chase. Cora returns to the sea and is accepted by their parents, while Maris chooses to stay on land—wearing Dorian's mother's jade ring on her left hand and her sister's pearl bracelet on her wrist, she has finally captured what she truly sought.
