Irish fisherman Syracuse (Colin Farrell) catches a drowning woman (Alicja Bachleda) in his net. She has no memory of who she is, tells him to call her Ondine, and refuses to be seen by anyone other than Syracuse. He allows her to live in a vacant house left by his deceased mother, whom he describes as also being secretive and not liking people. To entertain his daughter Annie (Alison Barry), who is wheelchair-bound due to kidney failure, he makes up a story about a fisherman catching a woman in his net, but Annie begins to believe his story and thinks that Ondine may be a selkie.
Syracuse brings Annie home after her kidney transplant. Annie asks about Ondine, but all Syracuse will tell her is that Ondine has gone home. Annie assumes that she went back to her "selkie husband" but assures Syracuse that she will return because she left something (her seal coat) here. As Annie is skimming through the TV channels, Syracuse suddenly hears the song that Ondine sang when on his boat. Realizing that it's a real song, not a selkie song, Syracuse returns to the island where he left Ondine. He finds her lying on the rocks and asks her to tell him the truth about herself. At first, she lapses into the story about being a selkie, but he presses her for the real truth, so she explains that she was a prisoner and a drug mule from Romania who was forced to dive overboard with a backpack of heroin when the coast guard boarded their boat. She swam until she could swim no more then started to sink, thinking she was going to die, until Syracuse's net brought her up. Her real name is Joanna. When Syracuse and Joanna return home, they find Annie being held by Ondine's "selkie husband" Vladic (Emil Hostina). An accomplice holds Syracuse at gunpoint while Vladic slaps Joanna around until she tells him where she buried her seal coat, which is actually the bag of heroin. When they find the bag missing, Annie admits that she moved it into one of Syracuse's lobster traps. While they are hauling in the trap, Annie and Syracuse push Vladic and his accomplice into the water. Vladic drowns. In order to prevent Joanna from being deported, Syracuse and Joanna marry, much to Annie's delight.