Dr. Baines' genetically altered piranha are virtually unstoppable - they can survive in salt water and they thrive on human flesh.

When Maggie and Paul accidentally release them into the Lost River, these ferocious predators threaten to destroy everything and everyone in their path.

With only a rickety raft to keep the piranha at bay Maggie and Paul must reach Lost River Lake in time to warn its residents and close its dam before the vicious piranha reach the ocean and spawn.