Following the catastrophic carnage at Lake Victoria, prehistoric, bloodthirsty piranhas migrate through underground water systems, seeking a new feeding ground. They infiltrate the newly constructed, state-of-the-art water park, "The Big Wet," managed by the greedy and unscrupulous Big Dave. Unbeknownst to the patrons, Dave knowingly ignores the impending threat to maximize opening weekend profits, driven purely by financial desperation and hedonistic desires.Maddy, Big Dave’s stepdaughter and a dedicated marine biology student, returns home hoping to convince her mother to sell the struggling park. Upon discovering strange, aggressive fish in the local water supply, Maddy teams up with her ex-boyfriend Barry and her friend Ashley to investigate the hazard. They quickly realize the mutant piranhas have adapted to thrive in chlorinated swimming pools and are multiplying at an alarming rate.Meanwhile, David Hasselhoff is hired as the park's
celebrity guest lifeguard, adding a layer of ironic vanity to the impending chaos. As the grand opening commences, thousands of unsuspecting, scantily clad teenagers flood the park, eager to party and enjoy the slides. Maddy tries desperately to warn the attendees, but Dave silences her, dismissing the danger as a hoax to protect his investment.The dam soon bursts, unleashing a torrent of flesh-eating horrors directly into the massive wave pools and water slides. Pandemonium erupts as the piranhas attack the crowd in a frenzied feeding frenzy, severing limbs and creating a chaotic sea of blood. Parkgoers attempt to flee, but are trapped by locked gates and surging crowds, turning the water park into a claustrophobic
slaughterhouse.Maddy, Barry, and local law enforcement fight valiantly to rescue survivors, utilizing whatever makeshift weapons are at hand. In a chaotic battle against the aquatic swarm, Hasselhoff heroically attempts to save stranded swimmers using his iconic slow-motion rescue tactics, though he becomes a target himself. Maddy races against time to shut off the park’s main water valves and poison the pool system. After a tense struggle, she manages to eradicate the immediate school of piranhas plaguing the park.In the aftermath, the few remaining survivors survey the devastated, blood-soaked grounds. However, the final moments reveal a lingering, terrifying twist: the resilient fish have already evolved the ability to survive on land, leaping out of the water to pursue retreating survivors, proving that the prehistoric nightmare is far from over.