After the events at Lake Victoria, the prehistoric school of blood-thirsty piranhas make their way into a newly opened waterpark, "The Big Wet Water Park," through an underground river. The park's co-owner, Maddy, finds herself at the center of the action as she returns to her hometown during summer break from school. Unbeknownst to her, her stepfather, Chet, has transformed the family-owned water park into an adult-themed attraction, complete with water strippers and outlandish events, in a greedy bid for financial success.

Maddy reconnects with her childhood friends and learns of the irregularities in the water park's plumbing system. Local water expert, Barry, discovers the presence of the piranhas right after Maddy's suspicions begin to mount. With the impending launch of the park's grand opening and a slew of visitors ready to dive in, Maddy faces the struggle of convincing the local authorities and her money-driven stepfather of the lurking underwater threat. The situation intensifies as the safety of the oblivious party-goers hangs in the balance, and the carnivorous fish start finding their way to the unsuspecting public.