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The film begins with a family of five, mother Elaine (Eva Birthistle), father Jonah (Stephen Campbell Moore), teenage daughter Casey (Hannah Tointon), and the two younger children Miranda (Eva Sayer) and Paulie (William Howes) heading up to a large house to spend New Year with some relatives. When they arrive, they are greeted by Elaine's sister Chloe (Rachel Shelley), her husband Robbie (Jeremy Sheffield), and their two children; Leah (Raffiella Brooks) and Nicky (Jake Hathaway).

Paulie, who has been feeling unwell for the journey is sick outside the house, so Elaine sees to him. While she looks after him, she tells Casey to carry the bags into the house and Casey asks her if she has heard of contraception. Inside, the children are enjoying themselves, except for Casey, who is annoyed that she didn't get to go to her friend's party. Chloe then gets annoyed because Nicky is playing with a toy gun that Casey got him for Christmas. She then continues moaning about her mother, who called at 11pm wanting to speak to the children, and tells them that she hasn't invited her mother over for New Year. Paulie climbs up on to the table, calling for Elaine, and when Jonah picks him up, he hits him in the face.

Later, when putting the children to bed, Chloe discusses the MMR jabs with Elaine as Jonah tells Robbie about his job. Casey then asks Robbie where she can get reception for her phone, and Robbie shows her because he is glad to escape the conversation. Miranda mentions that Casey isn't supposed to talk to her friends because Jonah has grounded her. Casey makes a remark about him, calling him Jonah, rather than Dad. Chloe tells the family about how she is home schooling the kids, and Jonah mentions that he is teaching Miranda to speak Mandarin.

Leah shows up and tells Chloe that Paulie scares her, and Miranda agrees to take her back to bed. Robbie takes Casey outside, and she leaves her friend a message on the phone, asking her to pick her up the next day. Casey hears the cat, and walks around to look for it, but she can't find it, so she re-enters the house. Robbie locks the door when she gets back in. Later on, Leah coughs something up and wipes it on her pillow, meanwhile, Paulie appears at the foot of his parents' bed, silently watching them.

The next day, everyone has a snowball fight, while Casey talks to her friend. She arrives back and asks Robbie if he has seen the cat, and he says it might be in the woods. She walks past a tent, where Miranda refuses to let her in. Inside, the cat's collar is hanging up, indicating that the children have done something to it. More of the sick that Paulie had earlier is seen on the floor, indicating that Nicky is sick too.

Later, Paulie is playing with a sledge, and it falls down the hill, hitting Chloe in the leg, causing her to drop a tray and spill hot tea over Jonah. Jonah gets mad at Paulie, and spanks him, with Robbie telling him not to hit children. He then shouts at Casey for not looking after the children, even though it was not her responsibility. The children go to the bedroom, and Miranda tells Chloe that she doesn't want to be there. When Chloe leaves, Miranda tells the others to stop staring at her.

Robbie is smoking in the greenhouse, when Casey comes in, and shows him her tattoo. Chloe arrives, sees her with her shirt lifted up and glares at her. During dinner, Chloe deliberately causes an argument by mentioning Casey's tattoo, but Miranda starts screaming at everyone and all the children go mad, so Casey and Robbie go outside to look after them. While there, she goes to the to the other side of the hill to meet her friend, but falls down the hill and hits her head. The kids put Robbie on the sledge, but it crashes, tearing skin off his head.

Inside the house, Jonah tries to get Paulie to go upstairs, but Paulie stabs him in the arm with a kitchen knife, before running away. Chloe finds Robbie, dead and shrieks at Casey to find her children, so Elaine heads out into the woods to find them, and bumps into Casey. Casey tells her she went to meet her friend when the accident happened, but she came back. Her friend texts her, but she sees Leah in the woods. She goes to find Leah, but trips over into some more sick and screams.

Elaine takes Chloe back into the house, then goes outside to find Robbie's body missing, finding it inside the tent. She sees Paulie on the climbing frame, and climbs up to rescue him, but her high heel slips and she falls, breaking her leg. Casey rescues her and drags her into the greenhouse, but the children begin throwing stones through the windows. Casey makes her a splint to help her walk, but then they hear Paulie trying to get inside. He crawls in, and attacks them, but Casey pushes a shelving unit on to him, before grabbing a knife to defend herself. Elaine tells her to stop, as Chloe comes outside. Elaine tells Casey not to let Chloe inside the tent, but Casey initially refuses to leave her mother to save her hated aunt.

Chloe enters the tent, but Leah (who is inside) runs past her, and begins to cut at the tent. Casey opens it up and rescues Chloe, but the evil bitch blames her for everything going on, and locks her outside. She enters the greenhouse, and tackles her brother to the ground. He picks up a pair of scissors and tries to stab her, but Elaine grabs him and he falls on to a shard of glass. Jonah enters to find Paulie dead.

Inside the house, Elaine tells Jonah what happened, and he blames her for the accident. Chloe calls Elaine and Casey monsters. Chloe tries to get out to find her children, and Casey tries to stop her. Jonah grabs Casey and throws her on to the sofa. Chloe tells Jonah that Casey needs to stay away from her family. Casey tells Elaine that it wasn't her fault, and the sickness is making the children go mad.

In the woods, Chloe moans and rants at Jonah that Casey is in the house and if the kids go back there, she will abuse them. Casey blocks the doors up and locks the windows, so no one can get in. Chloe then finds her children, who kill her by stabbing her in the eye with a crayon. Meanwhile, Casey finds Miranda upstairs, sabotaging the phone lines, and tries to strangle her. Jonah arrives, pulls her off and hits her with a toy phonebox before locking her in. Casey shouts for him, calling him Dad. Miranda runs downstairs and tells Elaine that Casey was hurting her, and Jonah leaves her on the sofa, before getting into Chloe's car with Miranda and driving away.

Casey shouts for Elaine to lock the back door, so the kids don't get in, and she locks it before climbing the stairs, but they get in through the cat flap. She tries to break down the door, but she can't. As Elaine gets outside the door, the children arrive, and she can't defend herself against them because they are children. Casey manages to kick a hole in the door, and grabs Nicky's head, impaling it on a piece of wood. As she is about to stab Leah, Elaine tells her not to.

Elaine starts her car and drives down the street, until she sees Chloe's car parked at the side of the road. Casey gets out to investigate and finds a hole in the windscreen. She walks around the car and finds Jonah buried under some snow, and starts to be sick. Suddenly, Elaine sees Miranda rushing towards Casey and urges her to move. Elaine accelerates towards her younger daughter, and hits her, killing her instantly. Casey gets back into the car, and they drive away, as other infected children, including Leah, appear in the woods, and the film ends with us wondering whether Casey now has the infection.