Sergeant Harry Wells leads a team of British soldiers on a routine expedition to the Scottish Highlands. The six men would rather be at home watching a game, but they are even more dismayed when a carcass lands on their campfire. The next morning, they happen upon a severely injured Captain Richard Ryan and the bloody remains of his squadron.

While searching for missing teenagers, novice skip tracer Maggie McKeown and local town boozer Paul Grogan stumble upon a top-secret Army laboratory conducting genetic research on piranha fish for the purpose of developing biological warfare. When the deadly eating machines are accidently released from the compound, they're soon headed downstream and consuming everything, and anything, in their path.