In Reaper Man, Death develops a personality and begins a mortal life as farmhand Bill Door, starting a backlash of unnatural events including an undead wizard, poltergeist activity, and animated objects. Worst of all, a dangerous hive mall is hatched from "city eggs." Ultimately, the mall is fought and destroyed. Through forging friendships in the mortal realm, Bill Door determines to fight Death, and he defeats the new Death and again takes on the role.

As the novel begins, Death, the grim reaper, is fired for having developed a personality. In exchange for his loyal service, Death is given a small measure of mortal lifetime and allowed to keep his horse Binky. Meanwhile, Windle Poons, a 130-year-old wizard, dies and returns as a zombie when Death does not reap his soul. His former wizard colleagues make several attempts to banish or destroy Windle. Frustrated, the wizards of the Unseen University, with Windle's full cooperation, bury the undead wizard at a crossroads. Alone in his coffin, Windle finds an ad for "The Fresh Start Club," a support group for undead. Hopeful, Windle digs his way out.

Death travels to Discworld to live as "Bill Door", working as a farmhand for an old woman named Miss Renata Flitworth. Meanwhile, with Death off the job, a build-up of life-force causes the city of Ankh-Morpork to experience strange poltergeist activity, as objects take on a life off their own. The various guilds of Ankh-Morpork are collectively unable to explain the strange spiritual activity.

The wizards attempt to conjure Death, but instead only see a gray robed figure who apologizes for the recent inconveniences. Elsewhere, Windle Poons arrives at The Fresh Start Club, finding sympathy in their ranks. Bill Door, meanwhile, proves an adapt farmhand and quickly makes friends. Miss Flitworth realizes that Bill is Death. Bill Door explains that he was Death, but is no longer and will soon die himself. Hearing that Bill's replacement is likely to be terrible, Miss Flitworth encourages Bill to fight for his life. Bill resolves to fight Death when it comes for him.

Among the odd events, strange snowglobes have been appearing and, thanks to a street peddler named Throat, soon circulate across the city. Windle visits Mrs. Cake, a spiritual medium, where he learns that the snowglobes are "city eggs." The wizards of the Unseen University soon find themselves inundated with what the "city eggs" hatch into: animated trolleys. They follow the trolleys back to a large pyramid made of sticky orange marble, a newborn hive mall. Meanwhile, Bill Door faces the new incarnation of Death, managing to dispatch the entity with the help of Miss Flitworth. In doing so, Bill Door again becomes Death. Later, Windle and the Fresh Start Club rescue the wizards from the depths of the hive mall, disabling the hive queen in the process.

Everyone escapes the hive mall. The wizards annihilate the construct with a series of devastating spells. Death, at long last, reaps the soul of Windle Poons. Afterward, he comes for the life of Miss Flitworth, first showing her a night on the town and momentarily reuniting her with the ghost of her long lost love. Death allows a small piece of himself, the Death-Of-Rats, to exist independently of his persona.