The novel takes the form of a letter written by Dunstan Ramsay to the Headmaster of Colborne College, where Dunstan is a teacher. A recent article in the school newspaper has portrayed Dunstan as dull and boring, and he wishes to correct this perception by telling his strange, marvelous, and complicated life story to the headmaster.
He begins with the night in the small town of Deptford Ontario that Percy Boyd Staunton threw a snowball at him, but instead hit the head of Mary Dempster, who is pregnant. Mary then prematurely gives birth to her son Paul as a result of the accident. Her husband, Amasa Dempster, a devout Baptist minister is burdened by his wife after the accident, for the blow to the head makes her “simple.” Mrs. Ramsay, Dunstan’s mother, takes care of Mrs. Dempster, pitying her because she has become ill equipped for sensible domestic life and because her husband’s religion prevents him from thinking clearly and helping her.
Dunstan also takes care of Mary and looks after Paul after school. He comes to love Mrs. Dempster, partially because he feels responsible for her misfortune. Dunstan works in the library as a child, and develops a love of learning, fantastical stories, mythology, and magic. He teaches Paul magic tricks, but this backfires—Amasa is furious that Dunstan has corrupted his son with “gambling tricks” and bans Dunstan from his home.
One night Mary Dempster goes missing, and Dunstan is part of the search party. He discovers her having sex with a tramp, and when questioned, she serenely confirms the sex was consensual. Amasa takes her home, resigns from the church, and moves her to a small cottage where he keeps her tied up. Mrs. Ramsay’s sympathy for her is gone, and she forbids Dunstan to see her. Dunstan goes anyway, and the closer he grows to Mrs. Dempster the more he believes there is something saint-like and holy about her.
Willie, Dunstan’s brother, grows ill as a result of an old back injury, and one afternoon while Dunstan is looking after him, Willie stops breathing. He has no heartbeat, and Dunstan believes he is dead. He gets Mary Dempster, and she touches Willie and calls his name, and Willie wakes up. Dunstan tells the doctor and his parents what happened, but they call him foolish and wonder why he didn’t seek out a doctor instead of Mrs. Dempster. His mother is especially furious, and demands that Dunstan choose between her and Mrs. Dempster. He chooses to enlist in the army.
WWI has begun, and shortly before he is shipped out Dunstan develops a timid romantic relationship with Leola, who is Percy’s girlfriend, but Percy has been sent away to school because he was caught having sex with another young girl in town. Leola still loves Percy, but says she loves Dunstan, too.
Dunstan fares well in the war, remaining uninjured until one day he suffers a blow to his leg after taking out three German machine gun operators. As he lies on the battlefield, sure he will die, he sees a statue of the Madonna, but her face is the face of Mary Dempster.
Dunstan wakes up six months later from a coma, and finds he has lost his leg but has been awarded the Victorian Cross (the highest military honor awarded in Canada) and that both his parents have died in the influenza epidemic of 1918. He has a romantic relationship with his nurse, Diana Marfleet, but realizes she is too much like his mother and ends it on good terms. He goes back to Canada to find that Boy (Percy’s new name) and Leola are engaged. Dunstan gets a degree in history, becomes a teacher, and has a kind of adult friendship with Boy, who often complains to him about Leola’s inadequacies.
Meanwhile, Dunstan becomes interested in renaissance art and sainthood and travels around Europe seeking out new knowledge, leaning new languages, and writing books. He comes across a traveling circus and meets Paul Dempster, who has no interest in forming a relationship with his mother, who has gone insane since he left and Amasa died from the flu, and who is living with her Aunt Bertha.
Back at his school in Canada, Dunstan encounters the tramp who slept with Mary—his name is Joel Surgeoner, he is a charity worker, and he believes Mary’s act miraculously reformed him. Thus Dunstan comes to believe that Mary has performed three miracles: she has reformed the tramp, brought Willie back from the dead, and appeared to Dunstan on the battlefield.
When Aunt Bertha dies, Dunstan, who has visited her and Mary consistently since he heard of their whereabouts, is named Mary’s guardian. He has to put Mary up in a public hospital, and feels horrible about it, but continues to travel. During this time, Leola finds out Boy has been cheating on her, and she attempts suicide. She withers away for years until, during World War II, she dies of pneumonia. Dunstan continues to travel and have success, earning the respect and friendship of the Bollandists, a group of Jesuits who study Saints. His closest friend is Padre Blazon, who encourages him to keep investigating Mary Dempster’s sainthood.
Dunstan takes a six-month leave to go to Mexico, where he meets Paul again, who is now a successful and famous magician. Dunstan is asked to write the autobiography of Eisengrim (Paul’s stage name). During this time Dunstan has a sexual encounter with a hideous, devilish woman named Liesl, who tells him he is not living life and has yet to accept himself as a human being. This encounter is later remember by Dunstan as his encounter with the Devil, and he believes it was a crucial and formative event in his life—Padre Blazon will tell him that a friendship with the Devil is indeed a good thing. Dunstan finishes the book about Eisengrim after this encounter, and it is a huge success.
Dunstan borrows money from Paul to move Mary to a private hospital. He stupidly tells her that her son is still alive, and this drives her mad. She thinks Dunstan is keeping Paul away from her, and Dunstan can no longer see her without upsetting her. When Mary dies, Dunstan feels it is his fault and weeps for the first time since childhood. She is cremated, and Dunstan keeps the ashes in his room.
Eventually, Eisengrim brings his show to Canada, and performs at Colborne College. Dunstan introduces Boy to Eisengrim. The three go back to Dunstan’s for a drink, and it is revealed that Boy doesn’t remember the Dempsters ever living in Deptford. Dunstan reveals to story of the snowball to Paul, and Boy says Dunstan has made a big deal out of nothing. Dunstan shows Boy the stone that was enclosed in the snowball—he has kept it as a paperweight all these years. Paul and Boy leave together, and Boy is found dead in his car the next day, with the stone placed carefully in his mouth.
At the next performance of Eisengrim’s show, an audience member asks the “brazen head”—the center of a fortune telling illusion put on by Eisengrim—who killed Boy Staunton. Liesl, the voice of the head, gives a cryptic answer, which Dunstan understands as implicating Boy, Paul, Mary Dempster, and him (Dunstan), in Boy’s death. Dunstan has a heart attack. When he awakens in the hospital, there is a note from Liesl, apologizing for causing him trouble and inviting him to live out the rest of his days with Eisengrim’s crew on tour. The letter to the headmaster concludes with this note.