Every year the population of sleepy Lake Havasu explodes from 5,000 to 50,000 for a single, wild weekend - the 4th of July, a riot of sun, drunken fun and sex-crazed mayhem. But this year, there's something more to worry about than hangovers and complaints from local old timers. Havasu sits in the crater formed by a prehistoric volcanic eruption, and when earth tremors tear open a crack in the lake floor, all hell breaks through. Piranhas - a million ravenous, razor-toothed monsters, unchanged since the dawn of time. Unstoppable killing machines acting blindly under one primeval impulse: to hunt down anything that moves and strip it to the raw, bleeding bone. In seconds. As bodies start disappearing, beautiful, feisty dock manager Julie Tupper realizes the horrifying truth. The idyllic little town she calls home is facing unthinkable carnage, no one wants to listen... and her diving expert fiancé has gone missing on the lake.