Dunstan Ramsay, an aging history teacher at Colborne College, becomes enraged by the patronizing tone of a newspaper article announcing his recent retirement, which appears to portray him as an unremarkable old man with no notable accomplishments to his name. Hoping to prove that he has lived a worthwhile and fulfilling life, Ramsay pens an indignant letter to the school's headmaster relating the story of his life, beginning with a childhood memory of an incident that occurred in his hometown of Deptford, Ontario in December 1908.
During a quarrel with a ten-year-old Ramsay (then known as "Dunstable Ramsay"), Ramsay’s wealthy friend Percy Boyd Staunton angrily hurls a snowball (inside of which is hidden a stone) at him, but accidentally hits his heavily pregnant neighbor Mary Dempster, causing her to prematurely give birth to a sickly child called "Paul". Apparently afflicted with severe mental trauma by the incident, Mrs. Dempster's behavior grows progressively more erratic until she is ostracized from polite society after being found having sex with a homeless tramp in a gravel pit, leading Paul Dempster to become an outcast in the village. While Ramsay takes pity on Paul and often keeps him company, Staunton refuses to take responsibility for throwing the snowball. The rift between the two deepens after Staunton begins a romantic relationship with Ramsay's crush Leola Cruikshank.
When Ramsay's gravely injured brother Willie apparently makes a miraculous recovery after Mrs. Dempster prays at his bedside, Ramsay comes to suspect that Mrs. Dempster is capable of performing miracles, which is seemingly confirmed after Ramsay himself has a vision of her shortly before miraculously surviving an artillery blast at the Battle of Ypres in World War I, losing his left leg in the process. Upon awakening in a military hospital from a six-month coma, he learns that he was initially presumed dead and posthumously won a Victoria Cross, and that his parents died from Spanish Influenza before learning that he was still alive. While recovering in the hospital, Ramsay has an affair with nurse Diana Marfleet, but breaks up with her after rejecting her marriage proposal, prompting Diana to playfully nickname him "Dunstan" after the 10th-century English saint who supposedly resisted the temptations of the Devil; this conversation later inspires Ramsay to have his first name legally changed to "Dunstan". Upon returning to Deptford, he learns that Staunton has become engaged to Leola, while Mrs. Dempster has been taken in by a relative after apparently going insane, and Paul Dempster has run away from home to join the circus.
After becoming a schoolteacher, Ramsay earns a reputation as an eccentric due to his interest in hagiology (the study of saints). Meanwhile, Staunton—now known as "Boy," shortened from his middle name—becomes a fabulously wealthy businessman. Despite tacitly resenting Boy for his money and status, Ramsay maintains an uneasy friendship with him and Leola, often accepting his financial assistance. Later, Ramsay becomes convinced that Mrs. Dempster is a saint following a chance encounter with Joel Surgeoner—the man who had sex with her in the gravel pit—who miraculously turned his life around after his sexual encounter with her. After successfully tracking Mrs. Dempster to Toronto, Ramsay offers to become her caretaker.
Following the birth of her son David, Leola becomes increasingly unhappy with her marriage to Boy, finding herself unable to adjust to high-society life due to her provincial upbringing. The Stauntons' marital difficulties culminate in Leola unsuccessfully attempting suicide on Christmas Eve in 1936 after a fight with Boy. When Leola dies of pneumonia a few years later, Ramsay suspects that she intentionally brought about her death by leaving her window open.
Ramsay's deepening obsession with hagiology leads him to travel to Europe to meet with the Bollandists (a society of Jesuit scholars who chronicle the lives of saints) after they agree to publish one of his articles. During his trip, he develops a close relationship with elderly Jesuit priest Padre Blazon, who specializes in chronicling the earthly side of saints' lives, believing that most saints are much more flawed and human than history might choose to remember them.
While in Mexico City on a six-month sabbatical from Colborne College, Ramsay attends a magic show put on by the mysterious illusionist Magnus Eisengrim, who is revealed to be an adult Paul Dempster. Intrigued by Eisengrim's spectacular illusions, Ramsay joins his entourage as he tours the world with his magic act, and gradually becomes close to Eisengrim's wealthy patroness Liesl, an eccentric woman with a bizarre androgynous appearance. Liesl, who becomes Ramsay's lover, senses that he has never been truly happy, having spent most of his life being overshadowed by other people whose lives have intersected with his own. To help him make sense of his role in the world, Liesl suggests that Ramsay is fated to play the part of "fifth business," a term for a supporting player in a stage show whose role can’t be easily classified, but nonetheless plays a vital role in resolving the plot.
Ramsay's recollections ultimately reach their climax in 1968 after Ramsay and Eisengrim both cross paths with Boy following a show in Toronto. In a tense conversation, Eisengrim reveals his true identity to Boy, and Ramsay tells Eisengrim about the events in December 1908 that led to his premature birth. Recalling the incident, Ramsay states that the snowball that Boy threw at Mrs. Dempster had a rock concealed in it, and claims that he still has the rock. Boy, however, still refuses to admit to throwing the snowball, denying any responsibility for Mrs. Dempster's misfortunes. After Boy and Eisengrim storm out of the room, Ramsay finds the rock missing.
Hours later, Boy is found dead in his car after apparently driving into a river, leaving the police unsure whether his death was murder or suicide. Curiously, a stone is found placed in his mouth, which Ramsay believes to be the rock that Boy threw at Mrs. Dempster as a child. Later, while watching a fortune-telling display at Eisengrim's magic show, Ramsay collapses from a sudden heart attack after someone in the audience cries out "Who killed Boy Staunton?" Onstage, the fortune-telling "Brazen Head" cryptically replies that he was killed by five people: by himself, by the woman he knew, by the woman he did not know, by the man who granted his inmost wish, and by "the inevitable fifth, who was keeper of his conscience and the keeper of the stone."
With that, Ramsay concludes the story of his life, saying only, "And that, headmaster, is all I have to tell you."
