Part 1: Mrs Dempster
Dunstan Ramsay, the novel's narrator, is not happy. He is a retiring professor of History with over forty years of distinguished teaching to look back on, and is annoyed that the school (Colborne College) has recently published a condescending article, implying that all he did in life was associated with the school. He is writing to the current headmaster, to insist that he has not simply lived the life of a lonely bachelor academic, but instead has explored a life of myth and wonder. The novel takes the form of his memoir.
Dunstan begins his memoir with his childhood in Deptford, Ontario, in which days he went by the name Dunstable. Though a seemingly quaint small town, Deptford has an ugliness in its conformity and sanctimony. The first event of the story is also the one that stays with Dunstan the rest of his life. One day, he is dodging snowballs thrown by his friend and nemesis Percy Boyd Staunton, and one accidentally hits pregnant Mrs. Dempster, wife to the meek village preacher. She crumples to the ground, and soon goes into premature labor.
Mrs. Ramsay, Dunstan’s mother, aids in the difficult delivery of her baby, Paul Dempster. Though the boy survives, he is small and Mrs. Dempster suffers some brain trauma that leaves her 'simple' in the eyes of the town. Dunstan is henceforth plagued by his conscience; he feels directly responsible for the tragedy, yet tells nobody about his role. This guilt is exacerbated when Mrs. Ramsay assigns Dunstan as the Dempster's personal caretaker, not trusting Mr. Dempster to handle it himself. Over time, Mrs. Dempster further alienates the town through her bizarre behavior, which is considered improper.
Meanwhile, Dunstan lives a relatively isolated life, with Paul Dempster as his only frequent friend. Through his after-school job at the Deptford library, he finds books on magic and saints, and becomes enamored of both subjects. He teaches Paul about magic, and the boy shows a great facility for it.
One night, Mary Dempster goes missing, and Dunstan joins the town search party. He is amongst the few who eventually find her in a dark gravel pit, engaged in carnal activity with a hobo. The town is appalled not only by her behavior but also by her seeming lack of remorse. She defends the sex as an act of charity: "he wanted it so badly" (48). Mr. Dempster is pressured to resign, the family moves to the outskirts of town, and she is kept locked in her house. Though Mrs. Ramsay forbids Dunstan to visit her, he does so secretly. The family is mocked and derided by everyone except for him.
Dunstan begins to see Mrs. Dempster as a saintly figure. Her simple logic and innate spirituality seems to radiate from her, and he basks in her presence. One day, Dunstan’s ailing brother Willie dies in front of him, and he for some reason fetches Mrs. Dempster, who seems to bring him back to life by laying her hands on him. He considers this her first 'miracle.' Of course, everyone else in town thinks him a fool. This time, when Mrs. Ramsay more forcefully forbids Dunstan contact, they have a terrible fight and he decides to enlist in the Canadian forces of World War I as a means to escape Deptford.
Before Dunstan departs, he becomes romantically involved with Leola Cruikshank, widely considered the prettiest girl in town and the fancy of Percy Boyd Staunton. It is a juvenile affair, but an enjoyable one.
Part 2: "I Am Born Again"
Dunstan leaves Deptford for the battlefields of Europe. While fighting in the fields of France, Dunstan single handedly destroys a German machine gun nest, although almost by accident. In the process, he is seriously wounded. Lying bloody near a ruined church, he turns his head to see the statue of the Virgin Mary, and is shocked to see that the figure has Mrs. Dempster's face right before he is struck unconscious by a falling flare. This becomes Mary Dempster’s second miracle.
Later, he wakes up in an English hospital, glad to be alive albeit with one leg missing. He befriends and falls in love with a pretty young nurse named Diana Marfleet, who is a little older than him, but also quite witty. Dunstan has his first sexual relationship with her, but decides to end the relationship for fear she might smother him as his mother did. Before he leaves, she rechristens him Dunstan from his birth name of Dunstable. He has the sense that he has been reborn, and is free to redefine himself.
Once healed, Dunstan returns to Deptford, wounded but also honored with the prestigious Victoria's Cross medal. A flu epidemic had swept through the town a few years before, killing his parents and Mr. Dempster in the process.
In Deptford, Dunstan learns that Paul Dempster had literally run away with the circus in order to escape his mother's reputation. She has been sent to live with an aunt, Bertha Shanklin, who lives near Toronto.
Dunstan sells his parents’ house and settles in Toronto to attend University. Dunstan’s rival, Percy Boyd Staunton, also shows up for school in Toronto, and brings his new wife Leola along.
Part 3: "My Fool-Saint"
Percy - who has renamed himself Boy Staunton - enjoys reminding Dunstan of his romantic loss (Leola) even though Dunstan has long since lost interest in her. Over time, Dunstan completes his degree in History, and settles into teaching at Colborne College for Boys.
Meanwhile, Boy shows his genius for making money by doubling his father’s fortune in short order. Soon enough, Boy joins the rich elite of Canada by building an empire in sugar and sugar related products. Unfortunately, Leola remains a small-town girl in many ways, and cannot ever live up to his expectations of a trophy wife.
Dunstan travels to find Mary Dempster in Weston. He endears himself to Aunt Bertha, and is permitted to regularly visit them. By now, Mary’s mind has completely gone, and she recognizes Dunstan only as a new friend.
Dunstan also becomes a familiar fixture around the Staunton mansion. Boy has taken a liking to Dunstan as his one confidant with knowledge of his past as Percy. In return, Boy provides Dunstan with stock tips that keep him financially comfortable, even during the Depression.
With that money, Dunstan travels to Europe, partly to study the saints who fascinate him, but largely to find the statue he saw on the battlefield. His journey is the first of many, and he slowly becomes an author and world authority on saints, despite being Protestant.
Now deep into his research of saints, Dunstan suspects that his own Mary Dempster is a bonafide saint. This supposition is strengthened when he discovers her third miracle (Catholic saints are required to perform three). After a philanthropist named Joel Surgeoner visits Colborne, Dunstan recognizes him as the hobo who Mrs. Dempster slept with so many years before. Later, Joel explains that that experience redirected his life, which he now devotes to charity.
Dunstan renews his study of saints with new vigor. While on one of his saint searching sojourns, he encounters Paul Dempster, who has honed his magic skills and now travels with a carnival. However, Paul wants nothing to do with his past or his mother.
Part 4: "Gyges and King Candaules"
Back in Toronto, Dunstan realizes that Boy is cruel towards Leola, who is sinking into a depression. He is also glib about their former relationship, sneakily forcing Dunstan to develop naked pictures of her.
Eventually, Aunt Bertha dies, and bequeaths her fortune to Dunstan provided he act as Mrs. Dempster's guardian. He is happy to oblige, but soon learns that Bertha's incompetent lawyer has squandered the money. Thus, Dunstan must put Mary in a public care mental facility or risk ruining his own finances.
Over in Europe, Dunstan meets a Jesuit priest named Padre Blazon, an eccentric whose attitudes on religious are devout but questioning. Through long conversations, he encourages Dunstan to become more active in his own life, and to embrace his natural dark side.
Part 5: "Liesl"
During World War II, Dunstan serves as interim headmaster of Colborne College, while Boy becomes an international figure through his faux-philanthropic efforts. While Boy is away, Leola dies in what seems a suicide, and Dunstan must manage the funeral.
When Boy, as head of Colborne's board, asks Dunstan to step down after the war, Dunstan asks for a sabbatical, and with it travels to Mexico to conduct some saint research. There, he meets Paul again. Paul has excelled even more as a magician, and has re-branded himself as Magnus Eisengrim. His show amazes Dunstan. He spends time with the troupe at the behest of Liesl, the bearded lady who manages the show's finances. She is a brilliant woman, and commissions Dunstan to write a fictional biography of the great magician.
Even more than Padre Blazon did, Liesl challenges Dunstan to embrace his inner potential and dark side. She describes him as "Fifth Business," an archetypal figure who always plays a supporting role and never takes the lead. Though he aggressively resists such classification - even getting into a sexualized brawl with her - he realizes she is a great mentor.
Part 6: "The Soirée of Illusions"
By now, Boy has almost no morals left. He practically ignores his children by marrying a new wife who promises to get him into politics.
Dunstan makes the mistake of telling Mrs. Dempster that he has found Paul. This brings back a flood of painful memories, which lead her to identify Dunstan and enemy and then slowly waste away. He never visits her again until she dies.
When Eisengrim visits Toronto to perform, Dunstan finally decides to put all his cards on the table. He introduces Boy and Paul to one another, but only Paul recalls the past. In their meeting, Dunstan reveals the secret of the snowball: the reason it hurt Mrs. Dempster so badly is that Percy had placed a rock inside it. Dunstan has kept the stone.
Uninterested in the past, Percy is unfazed, and dismisses any sense of culpability for that offense or for mocking Paul so mercilessly. At the end of the night, he gives Magnus a ride to his hotel.
The next day, Boy is found dead in the bay, clutching his steering wheel and with the stone in his mouth. That night, Dunstan attends the Eisengrim show, and realizes through one of the episodes that Paul had killed Boy. He has a heart attack on the spot.
After recovering, he retires and writes the letter to the headmaster, explaining his story and informing him that he is joining Liesl and Magnus with their carnival in Europe. In effect, he will no longer be only "Fifth Business."