Fisherman Matt Boyd is fishing in Lake Victoria, Arizona when a small earthquake hits, splitting the lake floor and causing a whirlpool. Boyd falls in and is eaten by a school of prehistoric piranhas that emerge from the chasm.
As spring break begins, Jake Forester reunites with his old crush Kelly and meets her arrogant boyfriend, Todd Dupree. Jake meets Derrick Jones, a sleazy pornographer as well as Danni, one of his actresses. Derrick convinces Jake to show him good spots on the lake for filming a pornographic movie. Jake's mother, Sheriff Julie Forester, searches for the missing Matt Boyd with Deputy Fallon. They find his mutilated body and contemplate closing the lake. However, this decision is made difficult with thousands of partying college students on spring break, which is important for bringing revenue to the small town. The next morning, a lone cliff diver is attacked and devoured by the piranhas.