Kale Brecht (Shia LaBeouf) and his father Daniel (Matt Craven) are enjoying a fishing trip. On their return home, their vehicle evades another and ends up in a multi-car accident. Kale is injured in the crash but Daniel is killed. One year later, near the end of the school year, Kale is still traumatized by his father's death. He is reprimanded about his school work by his Spanish teacher, Se単or Gutierrez (Rene Rivera); and when Gutierrez mentions Kale's father, Kale attacks him. For the assault, he is sentenced by a sympathetic judge to three months under house arrest with an ankle monitor and a proximity sensor. Detective Parker (Viola Davis) explains the do's and don'ts of the system to Kale and his mother Julie (Carrie-Ann Moss). The officer (Jose Pablo Cantillo) joyfully monitoring him follows suit by informing him that he is his teacher's cousin. Initially, he satiates his boredom by playing video games but, shortly thereafter, Julie logs out his subscriptions to the iTunes Music Store and Xbox Live and cuts the power cord of his television to get him to learn a lesson.
Kale's boredom leads him to watch the neighborhood, including the two neighbor boys who play pranks on him, his next-door neighbor Robert Turner (David Morse), and Ashley Carlson (Sarah Roemer), the new girl in town. One night, Kale becomes suspicious of Robert Turner, who returns home in a 1967 Ford Mustang with a dented fender that matches the description given on a news report of an errant serial killer. Kale's best friend Ronnie (Aaron Yoo) visits to spy on Ashley and, when they accidentally alert her to their hijinks, she joins them in spying on Turner. Kale observes a young woman Turner had picked up from a nightclub as she escapes the house in a panic, but later appears to leave the home in her car. Ashley throws a party and teases Kale, knowing he is watching. He plays music loudly, and when Ashley comes over, Kale admits that he has not only watched but has learned fine details about her by doing so. The two make out, distracting each other, while blood is shown splattering on Turner's windows.
Later, as Kale and Ashley watch, Turner is seen dragging a heavy bag to his garage, which Ashley claims to have noticed blood. Kale insists that Ronnie assist him in spying on Turner, eventually leading to Ronnie's breaking into Turner's garage with a camera. Though he confirms the bag has blood and hair in it, the garage door closes; and Ronnie flees into the house. Kale leaves to rescue him but alerts the police when he leaves his property; the police search the garage and show Kale the bag contains a roadkilled deer. Julie then goes across the street to talk to Turner, while Ronnie is in her kitchen, and Kale is watching the tape of Ronnie's running through Turner's house. Kale notices a dead body in plastic in a vent on the tape, as Turner knocks out Julie next door and then enters Kale's house, attacking Ronnie and Kale, binding and gagging Kale with duct tape. Turner reveals his plan to frame Kale for the murders before committing suicide.
As Turner has Kale writing a suicide letter to Ashley, she enters the home, and the two fight off Turner and flee the house. Kale trips the bracelet to alert the police, then enters Turner's home; Ashley goes to the police in person. In a hidden room, Kale finds evidence of Turner's previous murders, including their credentials, and a room with surgical tools and freezers of ice. Officer Gutierrez arrives at the scene, but he is killed when Turner breaks his neck. Proceeding to the basement of the house, Kale falls through the floor and finds his mother bound and gagged underneath the foundation. Turner suddenly appears from behind Julie and, in the ensuing confrontation, Kale fatally stabs Turner with a pair of gardening shears.
In the aftermath, Kale is shown having his ankle bracelet removed by the authorities for good behavior, getting back at the two boys from earlier pranks and making out with Ashley on his sofa, unaware that Ronnie is video taping them.

Question: How does Kale alert the police?
Answer: By leaving his house to try and save Ronnie.
Alternative answer: Tripping ankle bracelet