Willard Stiles is a socially awkward young man living a lonely life, dominated by his overbearing mother. His only solace comes in the form of a colony of rats he befriends, led by a particularly intelligent rat named Socrates. At his job, Willard faces constant belittlement from his boss, Al Martin, who had previously cheated Willard's father out of the company. As Willard becomes more attached to the rats, they begin to obey his commands, giving him a newfound sense of power and control that he lacked in his personal and professional life.

While navigating mounting stress from work and home, Willard discovers the potential to use his growing rat army to strike back at those who have tormented him. He trains the rats to execute his bidding, which begins to blur the line between affection and malice. As Willard's bond with the vermin grows, so does his determination to assert himself against his antagonists, leading to a series of increasingly dangerous events.