Sarah O’Neill and her shy,
arachnophobic son Christopher ("Chris") move to the
Irish countryside. While driving home, Sarah and Chris get into a car accident when she almost hits an old woman named Noreen Brady standing in the middle of the road. After returning home, Sarah and Chris have an argument about Chris’s father, and Chris runs into the forest. Sarah follows Chris and finds him near a large
sinkhole. At a dinner party, Sarah's friends recount the tale that Noreen believed her son James was not her real son and is rumored to have murdered him with a car. Later that night, Sarah awakens to sounds downstairs and finds Chris missing from his bedroom. Failing to find him, she calls the police, only to discover Chris standing in the doorway of his bedroom. Sarah visits a doctor the next day and is prescribed
sedatives. While driving Chris home from school, Sarah comes across Noreen. Noreen attacks the car and screams that the boy is not Sarah's son. Noreen's husband Des restrains Noreen, and Sarah drives away in shock. Later, Sarah visits the Bradys’ house and discovers Noreen has died by an apparent
suicide, by having her head buried in the dirt. During the funeral Sarah notices that all the mirrors in Noreen’s house are covered. Later, Des tells Sarah that Noreen believed their son James was an impostor; she said she could tell this by looking at the boy's reflection in a mirror. Des also reveals that it was he himself who killed James in a car accident. Sarah begins to note odd changes in Chris, when he becomes more sociable and develops a fondness for Sarah's cooking, which he previously hated. When Sarah confronts him about finding his
toy soldier in the forest during a run, he grows enraged and pushes the dinner table in an
uncharacteristic display of strength. One night Sarah observes Chris catching and eating a spider. During Chris's performance in the school talent show, Sarah becomes convinced the boy is not Chris. She hides a camera in Chris's room in an effort to monitor his behavior and later shows Des the video footage. He is dismissive but cannot reassure her that the boy in the video is her son. Sarah mixes her sedatives into Chris's food and later confronts him about his identity after Chris fails to recognize their favorite game. The imposter Chris attacks Sarah, knocking her unconscious. He buries Sarah's head in the ground but is knocked out by the sedatives soon after. Sarah frees herself and drags the imposter to the house's basement, where she uses a mirror to reveal the imposter to be an inhuman creature. Sarah locks him in the basement and flees to the forest's sinkhole. Sarah goes to the bottom of the sinkhole and eventually finds Chris still alive among dozens of
formless and faceless creatures. As the two escape, she is followed by one of the creatures, which takes Sarah's form. They manage to escape, and Sarah sets the house on fire with the imposter Chris still inside. Later, she and Chris start a new life in an unnamed city in an apartment filled with mirrors. She takes pictures of Chris from her window, one of which shows a blurred face. ==Cast==