Martha and Tomas Conroy relocate from London to a small Irish village where he grew up. In fact, it's his same childhood home. Martha is nearly nine months pregnant and is nervous because her previous child died of sudden infant death syndrome a few weeks old. During their first days in the new place Martha (Samantha Morton) meets the next door neighbor's girl, Daisy, who is rather odd. Another neighbor, an old man, hates her and throws rocks at her to keep her from snooping around his place. Then, Daisy's younger brother is found drowned on the nearby beach. Soon after this both her parents burn to death when their house catches fire. Since Daisy doesn't have another other relatives the Conroys feel sorry for her and take her in. When a child services worker says they can't keep her, but agrees to let the couple care for her another day, she ends up dying in a car crash that very night. And it's very clear that Daisy has something to do with it. The locals are unnerved by her, particularly when she's brought to school, and stories circulate that she's really a changeling (the real Daisy was swapped by Fairies years ago) and not human at all. It doesn't help that Martha lets her dress up in a fairy costume.
This is a very well-acted, moody horror film and one of the best of the "killer kid" sub-genre.