Willard Stiles (CRISPIN GLOVER) is a single and socially maladroit man. When not working for Frank Martin (R. LEE ERMEY) in the company his deceased father founded decades earlier, he spends his time caring for his frail and elderly mother, Mrs. Stiles (JACKIE BURROUGHS), with whom he still lives in their immense house.
When Willard learns that their basement is infested with rats, he sets out to exterminate them, but has a change of heart when he rescues a little white rat he names Socrates. For unknown reasons, the rat and his kind understand Willard and follow his verbal commands. After Mr. Martin belittles and berates him one more time, Willard gets the idea to use his army of rodents to make his boss pay for his meanness.
Seeing that his coworkers and new temp Cathryn (LAURA ELENA HARRING) enjoyed his prank, Willard revels in the anonymity of his newfound power. Yet, as the rat population in his house grows exponentially, things start to get out of control. That's particularly true when one large rat, Ben, begins to struggle with Socrates over leadership of the pack.
From that point on, and as Willard's employment, familial and residential well-being change or are endangered, it's only a matter of time before he utilizes his army in ways no one could have imagined.