Chyna Shepard is a college student visiting the family of her friend, Laura Templeton, for a long weekend. Chyna, who was abused and neglected by her mother as a child, finds that the Templeton house provides something she has yearned: acceptance. This comes to a violent end when serial killer Edgler Vess breaks into the house in the night and methodically kills all of the occupants except Laura and Chyna. After discovering that Laura has been tied up and raped, Chyna leaves, promising to return. Before she can intervene, Vess kills Laura and takes her to his motor home. Chyna hears Laura screaming and runs upstairs, intending to attack Vess with a knife. Unaware Laura is dead, Chyna sneaks aboard the motor home and finds her friend's corpse. Before she can escape, Vess drives away. Chyna hides in a back room, planning to escape at the earliest opportunity. When Vess stops at a gas station, she sneaks out of the motor home and looks for a payphone. Chyna secretly watches Vess boast to the gas station clerks that he is holding a young girl, Ariel, prisoner in his basement, before he kills them and drives away. She feels compelled to follow Vess and help free Ariel, taking a clerk's car. Chyna passes Vess while traveling through a state park and intentionally crashes her car into a redwood tree. While Vess gets out to investigate, Chyna sneaks on board the motor home. However, unbeknownst to Chyna, Vess glimpses her. Fascinated, he decides not to kill her immediately, wanting to see what she will do. Eventually, they arrive at Vess's remote house. Vess watches as Chyna leaves the motor home. She enters the house to find a catatonic Ariel locked in a room in the basement. Before she can free Ariel, Vess attacks Chyna in the kitchen, punching her unconscious before binding her with a chain. He taunts her for a while, revealing details about his past crimes. Obsessed with the "intensity" of any particular experience, sensory and existential, Vess styles himself as a "homicidal adventurer" and has killed continually since childhood. He offers to allow Chyna to live if she aids him in tormenting Ariel out of her catatonia. After Vess leaves for work, Chyna manages to break away from the table to which she is chained and slam her chair into a wall. She releases Ariel from her prison. Vess has trained a pack of deadly Dobermann pinschers to guard his property and kill anyone attempting to get in or out. Dressed in Vess's dog-training clothing, Chyna sprays ammonia on the dogs and makes it to the motor home with Ariel. Soon after, Chyna sees a police caron the road and pulls over to signal it, only to discover that the driver is Vess, the localsheriff. In the ensuing showdown, Chyna rams his police car, but he rolls clear and uses a shotgun to disable the motor home, causing it to tip over. Chyna and Ariel escape the wreck, but Vess catches up to them and knocks Chyna to the ground while Ariel continues on, distracting Vess long enough for Chyna to pull a lighter from her pocket. She uses it to ignite Vess's gasoline-soaked boot. She rolls away to safety just before the pool of gas surrounding Vess ignites. After catching up to Ariel, she turns and watches as Vess burns to death. A passing motorist stops to help them. Some months later, Chyna adopts Ariel and meets a nice man.
Several viewers of the film noticed striking similarities between the plot of the film High Tension and the plot of Intensity. When questioned at the Sundance Festival in 2004, the director Alexandre Aja acknowledged that he had read the novel and was aware of the similarities. On his website, Koontz stated that he was aware of the comparison but would not sue "because he found the film so puerile, so disgusting, and so intellectually bankrupt that he didn’t want the association with it that would inevitably come if he pursued an action against the filmmaker."
