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The movie is set in Ireland. There's a fisherman (and recovering alcoholic) named Syracuse, who people call "Circus" (or "Circus the Clown"; he says he was a clown for awhile, though I'm not totally sure if this is meant literally or metaphorically). He's divorced from a woman named Maura, who is apparently shacking up with a Scottish guy named Alex. Maura has custody of Syracuse's and her young daughter, Annie (who, aside from being a lovely and charming child, suffers from kidney failure, and spends much of her time in a wheelchair). Anyway, the movie starts with Syracuse out on his fishing boat, raising a net, which turns out to have a beautiful woman in it, who had nearly drowned. He resuscitates her, and wants to take her to a hospital, but she refuses. She doesn't want anyone to see her, so he takes her to the now-abandoned house where his late mother used to live.
Later, to help his daughter pass the time during one of her dialysis treatments, he starts telling her a story, about a fisherman who caught a woman in his net. But at that point he doesn't really know any more than that, so it's... not much of a story. Though he'll tell her a bit more, later. Meanwhile, she thinks the woman in the story might be a selkie (a creature from Scottish mythology). And... well, the woman, who calls herself Ondine, seems to bring Syracuse luck, as he catches a lot more lobsters, and fish, when she sings, than he usually does. And... Annie suspects the story her dad made up was real, so she eventually secretly follows him, and meets Ondine.
Here's the thing about this movie: well, I'm calling it "quirky," but it's got enough realism about it that you can't really think of it as a fantasy. Which leads one to assume Ondine probably isn't really a selkie, but you kind of can't help but wonder, especially since she basically plays along with Annie's belief. And there are plenty of things in the movie which are ambiguous... things which seem like they could fit with the myths, but which viewers could easily think up possible explanations for that are more realistic and more probable. Still, I ain't tellin' whether she is or not. And in fact, I guess I won't say any more about the plot's various twists.
I'll say that it's a reasonably romantic movie, and kind of amusing, and sweet, and a little scary, and well-acted, and beautiful. And Annie is probably the best thing about the movie, in terms of her humor and her relationship with both her father and Ondine. And... I guess that's all I have to say.