On the day of his twenty-seventh birthday, shy Willard Stiles, who works as a cashier in the factory once owned by his father, comes home to a surprise party thrown by his ailing, domineering mother Henrietta and her middle-aged friends. Upon learning that his mother has invited his boss, Al Martin, Willard is furious, as Martin wrested control of the business away from the Stiles family after the death of Willard’s father. Henrietta urges Willard to become more assertive so that he can advance in the company, but the dejected Willard storms out to the overgrown garden in the back of their huge, Victorian house. There, Willard sees a mother rat, and despite his initial fear, tosses her a crumb of his birthday cake. One afternoon, Willard is working in the garden, where he happily feeds the mother rat’s babies and discovers, much to his surprise, that he and the tiny animals like and understand one another. Henrietta insists that he call an exterminator to deal with the rats, one of which she had seen from a window, and after Willard states that they do not have money for a professional, he promises that he will drown them in the empty pool in the garden. When Willard arrives at work, he is alarmed to learn that Martin has hired a temporary secretary, Joan, to help him with the backlog of invoices. Fearing for his job security, Willard is suspicious of Joan, although she is attracted to the lonely young man. Later, Willard tricks the rats into walking down a ramp to an island in the middle of the empty pool, then fills the pool with water while they are eating. The rats’ distressed cries upset Willard, however, and he lowers the ramp for them to escape. As time passes, Willard spends more time with the mother rat and her ever-increasing brood, feeding and training them. He is pleased with their aptitude in learning words such as “food,” and in completing various tasks. An all-white rat catches his attention, and Willard, after naming him Socrates, teaches the intelligent, gentle animal many tricks. Socrates and Willard become inseparable, with Willard carrying the little animal in his pocket or on his shoulder, although he is careful to keep Henrietta from learning about his new friends. Willard also makes a special pet of a new rat, a wily, brown one whom he names Ben. Pressured by Henrietta, Willard asks Martin for a raise, but Martin insults him and instead offers to buy his home. Willard refuses, and later, exasperated by Martin’s bullying, takes the rats, whom he can now carry in a big satchel, to Martin’s anniversary party. Willard unleashes the rats on the veranda and watches triumphantly as the screeching partygoers scramble for safety. The next day, Henrietta dies, and later, after the funeral, Mr. Carlson, one of her friends, informs a distraught Willard that his mother left him only $1,500 and the heavily mortgaged house. Willard refuses to consider selling the house, however, and that night, moves the rats into the cellar. Socrates is allowed to sleep in Willard’s bedroom, and when the aggressive Ben makes his way upstairs also, Willard allows him to stay. Missing his pets during the day, Willard trains Socrates and Ben to stay quietly in the office storeroom, and after everyone leaves, the rats play on Willard’s desk while he works late. Willard has also treated himself to a used car with his inheritance money, and one afternoon, drives Joan to see his home. There, he finds a notice that the property will be sold for back taxes unless he pays $2,500, and despite his appeals to Henrietta’s friends for a loan, no one will help. At the office, Martin brags about a rich acquaintance, Walter T. Spencer, who is taking $4,000 in cash on a vacation the following day. That night, Willard takes two suitcases filled with rats to the Spencers’ residence, and as the couple sleeps, the rats begin chewing through their locked bedroom door. Upon hearing the noise, Walt opens the door and the rats swarm in, after which the terrified couple flees. Willard steals the cash from Walt’s money belt, then leaves with his pets. Later, Martin visits Willard’s home to appraise it, as he wants to turn it into an apartment building, and Willard overhears Martin commenting that Willard will be forced to sell if he is unemployed. Glum and worried, Willard takes out his frustrations on Ben, and the stubborn rat destroys the cane with which Willard swatted him. In the morning, Willard relents and allows Ben to accompany him and Socrates to the office. That day, both Joan and Willard are given their notices, and tragedy strikes when Martin’s secretary, Alice, sees Socrates in the storeroom. Because Ben is brown, he is able to hide, but the white rat is easily found, and Martin beats the animal to death. Although he is sickened, Willard cannot expose his secrets by preventing the slaughter, and that evening, must plead with Ben to trust him. Completely unbalanced, Willard fills his car with dozens and dozens of rats and drives to the office, where Martin is working late. Led by Ben, the rats follow Willard’s directions as he shrieks at Martin that he destroyed his life. When Martin hits Willard with the stick he used to kill Socrates, Willard orders the rats to “tear him up,” and Martin is attacked by the biting creatures. Screaming, Martin jumps out a window to his death. Horrified by what he has done, Willard says goodbye to Ben and runs off. At home, Willard drowns the remaining rats and seals the entrances to the house. The next day, Willard invites Joan over to eat, after which he tells her that because of his friendships with Socrates and her, he is no longer afraid of life. Their conversation is interrupted, however, when Willard spots Ben. Following him to the cellar, Willard sees swarms of rats waiting to follow Ben’s commands. Willard ushers Joan out, then promises Ben, who bites him, that he will feed him and his friends if Ben behaves. Ben sees as Willard surreptitiously tries to put rat poison into the food, however, and Willard is forced to chase Ben upstairs with a broom. The other rats gnaw through the cellar door and follow Willard upstairs to the attic. Screaming at Ben that he was good to him, Willard is soon covered by rats and killed while Ben watches.