Right from the beginning of the novel, Dunstan is aware of Mary Dempster. She is a tender young minister's wife who, according to Dunstan's mother and other village women, seems ill-suited to her position.

The pregnant Mary is out for an evening walk with her husband, Amasa, when Dunstan's friend Percy heaves a snowball. The snowball is meant for Dunstan, but Dunstan ducks and Mary is struck. This moment precipitates a lifelong burden for Dunstan. Mary goes into early labor and gives birth to Paul. It turns out that there was a stone in the snowball. Dunstan appropriates what should have been Percy's guilt, carrying the stone like a cross, and this leads him into an intense relationship with Mary and later with Paul.

Despite his mother's disapproval, Dunstan frequently sneaks into the Dempster home to help Mary. He develops a mentoring friendship with Paul, teaching him magic, which becomes a lifelong pursuit for Paul. When Dunstan's brother falls very ill, Dunstan enlists the aid of Mary, who revives him. Dunstan decides she has performed a miracle.

When Dunstan is wounded in the war, he has a vision of Mary in a statue of the Madonna. He is moved to pursue hagiography, the study of saints. Dunstan is convinced that Mary is a saint and makes it his life's work to seek sainthood for her.

Dunstan takes on the expense of Mary's care as she ages, and when he is reacquainted with Paul in later years, he tries to persuade him to visit and care for his mother. At the end of the novel, Dunstan finds himself in an encounter with Paul (who has taken the stage name Magnus Eisengrim) and Boy (formerly known as Percy). Dunstan tries to elicit an admission of guilt from Boy, but to no avail. Of the three of them who should have shared responsibility for Mary, only Dunstan has been faithful.