Plot
Social misfit Willard Stiles, who cares for his ill and fragile but verbally abusive mother Henrietta in a musty old mansion, is constantly humiliated and mercilessly taunted in front of his co-workers by his vicious and cruel boss, Frank Martin, who took over the Stiles family company after Willard's father Alfred committed suicide in 1995; Frank promised Stiles that Willard would have a job as long as she lived. After discovering and failing to exterminate a growing rat colony in the basement, Willard befriends and quickly becomes obsessed with a white rat he names Socrates, considering him his only real friend.
At work, Martin locks Willard in the elevator, though Willard's sympathetic co-worker Cathryn eventually releases him. That evening, he watches Socrates begin tearing up a newspaper, prompting him to train the now-expansive horde of rats, including Ben, a monstrously large rat (portrayed in the film by a Gambian pouched rat) who envies Willard's favoritism towards Socrates. After training the rats sufficiently, Willard takes them to Martin's home and orders them to chew up the tires on Martin's new Mercedes-Benz. While Willard initially rejects bringing along Ben with the other rats due to his size, the rat sneaks along anyway and chews a hole in the garage door to join the other rats in destroying all four tires of the Mercedes. A dog follows Willard in his hurried run from the house and he puts the dog in the bag before deciding to take it out after hearing it yelp when packed in with the rats. The next day at work, an on-time Willard gleefully watches Martin arrive late and tired. While Willard explains to the rats that they must move out, Henrietta overhears him, assuming he plans to get rid of her. Startled by noises from the basement, Willard finds her dead the next morning, having fallen down the basement stairs that allows the rats to come out from the basement, including a rebellious Ben. At the wake, Willard learns that Henrietta refinanced the family home to pay off Alfred's debts resulting in the house's payments falling behind and that the bank will likely foreclose upon the property.
In Henrietta's bedroom, Willard finds an envelope from the coroner's office containing the effects found on Alfred's body, including the still-bloody pocket knife he evidently committed suicide with. Distraught, Willard attempts suicide with the knife until Socrates stops him. Attempting to comfort Willard, Cathryn describes her own mother's death and gives him a pet cat, whose own mother helped Cathryn to grieve. Willard reluctantly takes it, with Socrates safely stowed in his pocket. In a scene set to the title song from the 1972 movie Ben , the rats (who have by now taken over the entire house) pursue and fatally overpower the cat.
Desperately lonely, Willard starts bringing Socrates to work with him. Despite a contract written by Alfred stipulating that he remain employed by the family company, Willard finds a note at his desk from Martin declaring that he is being fired. While Willard desperately pleads with Martin to let him keep his job, Martin's secretary, Barbara Leach, discovers Socrates in the supply room. Her screams alert Martin, who fatally bludgeons Socrates to death while Willard watches helplessly. Fed up with all the torment, Willard finally snaps and hatches a plan to avenge Socrates' death with Ben. Loading the numerous rats into a company van, he confronts Martin and orders them to swarm upon Martin and kill him.
At home, Willard kills the remaining colony before ratproofing his entire house. Exhausted after the night's events, he is finally awakened by the doorbell. Terrified by the shadows of two policemen, he remains standing in the hallway until the evening, seemingly feverish. Cathryn appears, informing Willard that Martin's body was found and that rumors had arisen that he was either murdered or eaten by animals. Coming face-to-face with Ben, Willard tries to leave the house before realizing that the leftover rats have chewed out his car tires. Accosted by the two policemen, Willard retreats into the house and frantically attempts to prevent the rats from entering. Trapping himself in the kitchen, Willard is confronted by Ben, whom he tries to kill with a rat trap.
Believing Willard is insane, and aghast at the rat infestation, the police leave to call Bellevue Hospital and the health department. When Cathryn tries to enter the house, they warn her against entering, asking whether she wants to be eaten alive. Horrified, she connects the rats to Martin's death and realizes Willard's complicity. Ben, even though severely injured, viciously attacks Willard as he tries to escape. The police officers and Cathryn look on as Willard, outlined in the upstairs window, kills Ben with his father's pocket knife.
The final scene reveals Willard in a psychiatric hospital, seemingly semi-catatonic and refusing to talk. A white rat appears in his cell, crawling into Willard's sleeve as Socrates used to. Believing that his friend has been reincarnated, Willard joyfully drops his semi-catatonic act and begins telling the rat his plans for an escape.
