After a few no-name characters are killed by the carnivorous piranhas, Maddy and the others are warned of the danger by crazy scientist Goodman (Christopher Lloyd). The grand opening of The Big Wet still goes ahead, with David Hasselhoff as "celebrity lifeguard" for the day. The piranhas arrive via the park's drainage system and begin feeding on all the partygoers.
Also hanging around is Deputy Fallon (Ving Rhames) from the first film, who is now legless and has a shotgun strapped to his titanium prosthetics, which he uses to start blowing away the fish.
The Hoff saves a little boy whom he earlier tried giving an autograph to; in the chaos, Chet tries driving away on a cart with the money, but is distracted by a girl´s bouncing breasts and decapitated by a flag wire.
Barry manages to drain all the pools, sucking away all the piranhas, but Maddy gets pulled down into one of the vents after Kyle chickens out and leaves her to die. She is saved by Barry after he overcomes his fear of water.
Another park employee, Big Dave (who earlier had a piranha lodged in his backside), pours gasoline and a lit cigarette into the drainage system, causing a huge explosion and blowing all piranhas sky high. Kyle gets speared through the eye by a falling trident.
Maddy gets a call from Goodman, who tells her that the piranhas are evolving and learning to walk on land. One of them does just that and decapitates the little boy who Hasselhoff had saved earlier, as the film ends.