One night at the Wet Valley Water Park, two teenagers are killed by a machete-wielding assailant. Some time later, the 2018 graduating class of Valley Hills High visits the water park, owned and supervised by Paul Wilkinson, for a weekend-long celebration of their graduation. Among those present are Josh, a member of a band called the Blades; Josh's bandmates Chad and Slim; Priscilla, Paul's wife; Alice, a graduate with whom Paul is cheating on Priscilla; Tommy, one of the park's employees; Conrad, an elderly
pool cleaner; and Kimberly, who is Tommy's girlfriend and Josh's ex-girlfriend. Rumours circulate amongst the students about Priscilla supposedly choosing one graduate to have sex with each year. Two drunk graduates go down one of the park's
water slides, and are injured when one impacts with a glass bottle held by the other. They leave in an
ambulance, and Paul orders Conrad to clean the pool without draining it. That night, Priscilla joins Josh's father, a
real estate developer named Michael, in a restaurant for dinner. Josh and Kimberly have sex in the showers, and are secretly recorded on video by another graduate named Phil. Later, the Blades perform a song in the park. During their performance, the video of Josh and Kimberly having sex is sent to the other graduates, including Tommy, who punches Josh and is fired from the park. Shortly thereafter, Michael tells Josh that he is negotiating ownership of the park with Paul. Elsewhere, an unseen individual installs a pair of large, metal blades, arranged in an "X" shape, in one of the park's water slides. The next day, Michael is fatally shot by an unseen attacker, and a boy uncovers a
portable music player in a sandy area of the park. Later that day, an annual competition begins: teams of three members are to race down one of three slides, and whichever team reaches the pool below fastest wins a cash prize. Tommy worries that one of the slides will collapse, but his concerns are ignored. The first set of teams each go down one of the slides. Alice and two other graduates go down the slide containing the blades and are cut to pieces. Tommy, while attempting to warn his fellow graduates not to go down the slides, is pushed down the weaponised slide; he becomes stuck against the blades and the bodies caught therein. The pool below begins to fill with blood and limbs from the weaponised slide, causing spectators to panic. Paul and Priscilla, unaware of the danger, continue to send teams down the slides, and eventually decide to slide down themselves. Josh unsuccessfully attempts to save Paul from sliding into the blades. Josh and Conrad both climb to the top of the slide to try to prevent other people from sliding down the slide but are pushed down the slide themselves. Following the massacre, a surviving Josh meets with a lawyer, who informs Josh that Michael left him his estate, and has him authorise the replacement of the Wet Valley Water Park with a
shopping complex. It is then revealed that the individual who installed the blades in the slide and killed Michael was Priscilla, who, 35 years earlier, witnessed her father die in the water park, and no one bothered to help him. In a
mid-credits scene, the boy with the portable music player surfaces in the bloody pool. ==Cast==