Taking the form of a first-person memoir, Fifth Business is the life story of Dunstable (later Dunstan) Ramsay, a retired school teacher whose life has been guided by the conviction that there are saints in the contemporary time period, and that his childhood neighbor is one such person. The novel’s title refers to a figure in an opera who is not directly involved in the action but exists only to observe and comment on it; clearly, Ramsay is such a figure. His story begins when, at the age of ten, he dodges a snowball thrown by Percy Boyd Staunton; the snowball hits Mary Dempster, the wife of the Baptist minister. The incident sends her into labor, and Paul Dempster is born eighty days early. Young Ramsay feels himself responsible.

The snowball and emergency childbirth bring about a change in Mrs. Dempster. She becomes unhealthily generous, disgracing her husband by having sex with a tramp. Later in the novel it is revealed that this act brings about a miraculous transformation in the tramp, who becomes an inner-city missionary. Her other miracles entail apparently bringing Willy Ramsay back from the point of death and, years later, appearing to Dunstable Ramsay on a statue of the Virgin Mary in a World War I battlefield. These miracles lead Ramsay to the belief that Mary Dempster is a saint.

Young Ramsay develops an interest in conjuring—stage magic—and introduces the art to Paul Dempster. Later, while Dunstable is fighting in the war, Paul runs away with a circus, eventually becoming a world-famous stage magician. Ramsay’s interest in magic connects with his interest in sainthood; his curiosity about saints has less to do with Christianity than with the supernatural in general. In fact, the New Testament is associated in his mind with the tales of the Arabian Nights. In Jungian fashion, he traces the parallels between Mary Magdalene and many similar figures in pseudohistory and mythology in search of their archetypal significance.

Ramsay returns from the war severely injured—he has lost a leg, and his body is scarred from burns. He is also a decorated hero. A girlfriend rechristens him Dunstan, after a saint who allegedly twisted the devil’s nose with a pair of tongs. He reconnects with his friend Percy Boyd Staunton, who has also renamed himself Boy Staunton; thus they are both “twice-born,” which in mythological terms sets them apart somewhat from normal people.

After earning two degrees, Ramsay becomes a teacher in a boarding school and takes annual “saint-hunting” trips to Europe; eventually he writes several significant books on saints and earns a reputation as a hagiographer. On one such trip, he visits a traveling carnival, where he is reunited with Paul Dempster, now a magician.

Many years later, Ramsay encounters Dempster yet again, this time under the stage name Magnus Eisengrim. The magician’s closing illusion dramatizes the union of Sacred and Profane Love in the Eternal Feminine. Ramsay agrees to travel with the show and write Eisengrim’s (Dempster’s) fictional autobiography. He has an encounter with the show’s backer, Liesl, that significantly parallels St. Dunstan’s encounter with the devil.

Eventually he introduces Boy Staunton to Dempster and tries to make Staunton accept responsibility for throwing the snow-covered stone that induced Mary Dempster’s early labor and apparently triggered her madness. Staunton and Dempster leave Ramsay’s home together, and the next morning, Staunton is found in his submerged automobile with the stone in his mouth, the implication being that Dempster, a hypnotist and an escape artist, has encouraged him to commit suicide, as he must have unconsciously wanted to do.