Jackie Ivers is a Los Angeles nurse who returns home to the small town of San Vicente to find that her friends and family have taken on bizarrely different personalities. Jackie notices that everyone who goes into the town's lake come out different...

A nerdy high school super whiz experiments with a chemical which will transform his guinea pig "Mr. Mumps" from a gentle pet into a ravenous monster. In a fit of rage against his tormentors at the high school, Vernon Potts goes on a killing spree, eliminating all of those who ever picked on him - the Gym Coach, the School Jock, The Creepy Janitor & his hated teacher, Ms. Grindstaff.