One night at the Wet Valley Water Park, a machete-wielding assailant cuts short the lives of two teenagers, setting a grim tone that threads through everything that follows. Years later, the 2018 graduating class of Valley Hills High returns for a weekend-long celebration, this time at a park that is owned and supervised by Paul Wilkinson. Among those present are Josh, a musician in a band called the Blades; Chad and Slim, Josh’s bandmates; 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 Kimberley, Tommy’s girlfriend and Josh’s ex. Rumors swirl among the students about Priscilla supposedly selecting one graduate to have sex with each year, a whisper that foreshadows darker motives to come.
Two drunk graduates dash down a water slide and are injured when one hits a glass bottle carried by the other; they are taken away by ambulance, and Paul orders Conrad to clean the pool without draining it. That night, Priscilla joins Michael, Josh’s father and a real estate developer, for dinner, deepening the complicated web of relationships around the park. In the showers, [Josh] and [Kimberley] have a private moment, which is secretly recorded on video by another graduate, Phil. Later, the Blades perform a set in the park, and during the show the video of Josh and Kimberley’s encounter is sent to the entire group, including [Tommy], who reacts with anger and is fired after punching Josh.
Not long after, Michael reveals that he is negotiating park ownership with Paul, a move that intensifies tensions behind the scenes. Elsewhere, an unseen figure installs a pair of large, metal blades arranged in an X shape inside one of the park’s slides. The following day, Michael is fatally shot by an unseen attacker, and a boy discovers a portable music player buried in the sand, a clue that will later tie back to Phil.
The annual event continues with a three-team race down the slides, and anxiety grows as concerns about safety are dismissed by those in charge. The first teams descend the slide with the blades, and tragedy erupts: Alice and two other graduates are cut to pieces as the deadly mechanism activates. As panic spreads, Tommy tries to warn everyone, but in the chaos he is pushed down the weaponized slide and becomes hopelessly entangled in the blades, his screams echoing as the pool fills with blood and dismembered remains. Paul and Priscilla, unaware of the danger, keep sending teams down, and even decide to ride the slides themselves. Josh attempts to intervene to save Paul, but his efforts fail; both Josh and Conrad scramble to the top to halt others, yet they are forcibly pushed down as the crowd roars in confusion and fear.
In the aftermath, a surviving Josh meets with a lawyer who explains that Michael left him a portion of the estate and directs him to authorize the park’s replacement with a shopping complex. A shocking revelation comes to light: the killer behind the blades and Michael’s death is Priscilla, who, 35 years earlier, had witnessed her father’s death at the same park and felt no one come to his aid. The tragedy reframes the entire night as a long-buried grievance surfaces with brutal purpose.
The film closes with a mid-credits scene that returns to the boy with the portable music player, who emerges again from the bloody pool, a haunting reminder of how the past bleeds into the present and reshapes every future choice.
