The book is based upon the premise that Dunstan Ramsay is writing a letter to the headmaster of the school he taught at for 40 years complaining that the article about his retirement made him sound insignificant. He then proceeds to write the book detailing his life, so that the headmaster understands that he is important.
Dunstable Ramsay was born in Deptford, Canada, a small religious town. He witnessed his friend, Percy Boyd Stauton, throw a snowball which struck a pregnant woman, Mary Dempster, causing her to go into labor prematurely. Dunstable then watches after Mrs. Dempster, the Baptist minister’s wife, and her baby, Paul, at his mother’s request. He grows quite close to Mary, even though she is considered insane by the town. As Paul grows older, Dunstable teaches him magic tricks, but when Reverend Dempster finds out, he bans Dunstable from seeing their family ever again. Then Mrs. Dempster goes missing, and Paul helps find her. They find her having sex with a tramp because he seemed to desperately want it. Mary is then further ostracized, and Reverend Dempster keeps her tied up in her home. Dunny begins to visit regularly, and their close friendship is renewed.
Dunny’s brother falls ill, and when Dunny is afraid he has died, he runs and gets Mary Dempster. Although the brother lives, Dunny’s mother is furious that he retrieved Mrs. Dempster and not a doctor or his mother. This fight leaves Dunny so angry that he enlists in the army. During training he has a short affair with Percy Boyd Stauton’s girlfriend, Leola. He then ships out.
While in the trenches of WWI, Dunny reads the Bible many times over. One night he is sent to raid a machine gun nest, and is wounded in the process. He successfully clears the nest, and then faints in front of a statue of the Virgin Mary.
He wakes in a hospital in Great Britain. His leg has been amputated, and he receives a Victorian Cross for heroism. He enters a relationship with his nurse, Diana, and she inspires him to get an education. Although she wanted to marry him, Dunstable decides to return to Canada alone for his education. Upon his return he discovers that both his parents and brother are dead and sells off their assets. He begins studying saints obsessively and reunites with Percy Boyd Staunton, who has changed his name to Boy Staunton. At this time, Dunstable changes his name to Dunstan. After completing school he begins teaching at a boarding school.
Boy Staunton has married Leola. While Boy keeps climbing the social ladder and has become a very powerful man, Leola has not outgrown her country upbringing. They have two kids, but the home is not happy. Dunstan often visits, but rarely defends Leola.
Dunstan happens upon Mary Dempster’s tramp, and finds him a new man. The tramp runs a charity. Dunstan considers this the second of Mrs. Dempster’s miracles, the first being bringing Dunstan’s brother back to life. He begins to think of her as a fool-saint. Dunstan then seeks her out and begins a new friendship with her.
While researching saints in Europe, Dunstan finds Paul Dempster travelling with a circus and performing magic tricks. He decides not to tell Mary Dempster for fear it will upset her.
Boy is now quite rich because he sells solace during the Depression. Christmas of one year, Leola discovers an affair Boy is having and attempts to seduce Dunstan. Dunstan however has no feeling for her other than pity. Leola is so upset, she attempts suicide, but fails.
The aunt caring for Mary Dempster dies, and leaves Mary to Dunstan’s care. Because he is short on funds, he places Mary in a public hospital.
Dunstan returns to Europe, this time visiting a group of Jesuits. He meets with Padre Blazon, who reminds Dunstan not to blame himself for Mary Dempster’s fate.
Leola dies, perhaps of her own choosing. Boy doesn’t come home to aid his family, so Dunstan takes care of the funeral and the children. Dunstan is named temporary Headmaster of his school after the previous dies unexpectedly. He remains Headmaster through WWII, until Boy, a member of the Board of Directors, asks him to step down. He does so under the condition that he may travel once more before resuming his post as a history teacher.
While traveling in Mexico, Dunstan sees Paul. Paul still performs as a magician; however he is much more accomplished now. Liesl, a member of Paul’s troupe, becomes a confidante for Dunstan. She never promises secrecy, but he tells her everything. He lusts over another troupe member, Faustina. When he sees her kissing Liesl, he becomes depressed. Then Liesl tries to seduce Dunstan, he responds by breaking her nose. When she returns, she claims she was provoking him so that he would release his pent up anger. Then he has sex with Liesl. It is at this point he realizes he is not the main character of his story, but “Fifth Business”.
Upon his return to Canada, Dunstan finally tells Mary Dempster that he has seen Paul. She is so upset that she is moved to a higher security ward. Shortly after being transferred to a private hospital, Mary dies.
Dunstan yet again visits Europe. Blazon approves of Dunstan’s self discoveries. Dunstan finds his Madonna statue.
Paul travels to Toronto with his show, and following the performance, he meets with Dunstan and Boy. All of the grudges and secrets of the book come out. Dunstan presents the two with the rock Boy hid in the snowball that hit Mary Dempster, but the rock goes missing. Dunstan suspects that Paul stole it. Boy refuses to admit any guilt in the incident. Boy is then found dead, under peculiar circumstances, with what appears to be the same rock in his mouth.
During a show of Paul’s an audience member asks “Who killed Boy Stauton?”, which gives Dunstan a heart attack. And so the letter ends.
