A young Henry Oldfield lives on a sheep farm in New Zealand, with his father and older brother, Angus. After witnessing his father's pride on Henry's natural ability at farming, Angus plays a cruel prank on him involving the bloody corpse of his pet sheep, just moments before Mrs. Mac, the farm's housekeeper, comes to tell the boys that their father has been killed in an accident. The combined shock of these two incidents leads Henry to develop a crippling phobia of sheep.

Fifteen years later, Henry (Nathan Meister) returns home to sell his share of the family farm to Angus (Peter Feeney). Unknown to Henry, Angus is carrying out secret genetic experiments that transform sheep from docile vegetarians into ferocious carnivores whose bite can transform a human into a bloodthirsty half-sheep monstrosity. While trying to expose Angus's experiments, a pair of environmental activists named Grant (Oliver Driver) and Experience (Danielle Mason) accidentally release a mutant lamb. As Grant and Experience flee from the scientists, the lamb bites Grant and infects him. The lamb then escapes into the fields and infects the flocks of sheep.