Maddy (Danielle Panabaker), a level-headed graduate student, envisions a happy summer working at the family-owned water park, The Big Wet Water Park in Cross Lake, Arizona. Celebrity lifeguard David Hasselhoff is on hand for opening day, but Maddy’s hopes are quickly dashed when blood thirsty piranhas invade the lake. Meanwhile, Barry (Matt Bush), Maddy’s fellow water park worker, despairs that Maddy will never view him as more than a friend.
When the danger is revealed, eccentric piranha expert Mr. Goodman (Christopher Lloyd) returns to give advice, as does survivor and porn cameraman Drew Cunningham (Paul Scheer). Meanwhile, a grizzled farmer (Gary Busey) searches the lake for the corpse of a prize cow.