Desperate for companionship, the repressed Willard befriends a group of rats that inhabit his late father's deteriorating mansion. In these furry creatures, Willard finds temporary refuge from daily abuse at the hands of his bedridden mother and his father's old partner, Frank. Soon it becomes clear that the brood of rodents is ready and willing to exact a vicious, deadly revenge on anyone who dares to bully their sensitive new master.
