Better Watch Out

At Christmas time, 17-year-old Ashley is babysitting precocious 12-year-old Luke Lerner. Luke has romantic feelings for Ashley and unsuccessfully attempts to seduce her while watching a horror film. Strange happenings outside put Ashley on edge, but it ultimately appears to be just Luke's best friend, Garrett, who has arrived to visit. Hearing a window break upstairs, the trio find a brick inscribed with "U leave and U die." Garrett panics and runs out the back door, but is apparently killed by an unknown shooter.
Shocked, Ashley and Luke run upstairs and hide in the attic. Ashley almost breaks her neck from a fall but Luke catches her. After they run into Luke's room and hide in his closet, a masked intruder armed with a shotgun walks in. Ashley recognizes the mask as one of Luke's and tells the intruder to take it off, revealing Garrett. Ashley realizes that Luke was hoping to seduce her by scaring her and staging a rescue. Angered, Ashley berates Luke before Luke knocks her unconscious.
Luke binds Ashley with duct tape, forcing her to play truth or dare. Her boyfriend Ricky arrives at the house after receiving a text message sent by Ashley earlier. When Ricky fails to find Ashley, he realizes that something is wrong. Luke tries to knock him out but fails. Attacking Luke, Ricky is threatened by Garrett with the shotgun. Luke knocks Ricky out and they tie him up beside Ashley.
While Ashley uses a shard of glass to cut herself free, Luke hits Ricky with a paint can, killing him. Horrified, Ashley threatens Luke at gunpoint, before realizing that the gun is empty. She tries to escape to a group of carollers outside, but Luke uses the brick tossed through the upstairs window to knock her out again before she can alert them.
Luke calls Ashley's ex-boyfriend, Jeremy, claiming that she wants him to write an apology letter. While Jeremy does so, Luke hangs him from a tree, making the apology look like a suicide note. Garrett has a change of heart and starts to free Ashley, but is killed by Luke. Luke stabs Ashley in the neck and stages the scene to frame Jeremy. Afterward, he goes to bed and waits for the return of his parents, who contact the police. Ashley survives, having placed duct tape over the stab wound to stop the bleeding. Luke watches from his bedroom window as Ashley gives him the finger before being wheeled into the ambulance.
In a mid-credits scene, Luke says to his mother: "Mom, I'm worried about Ashley. I think we should go to the hospital".