Based mainly on an epic poem of the twelfth century which was derived from a French chivalric romance, The Holy Sinner relates the lives of two legendary figures, the beautiful and noble twins Sibylla and Wiligis. Their mother having died at their birth, the two are devoted to each other and believe that no one else is equal to them in any way. Unable, therefore, to be parted in childhood and youth, the two become lovers when their father, Duke Grimald, dies. In time, Sibylla conceives a child. To do penance for their sinful attachment, Wiligis goes on a pilgrimage to Rome but dies before he achieves his goal. Sibylla, in the meantime, is taken to the home of the Sieur Eisengrein, a loyal baron and the counselor of their father. When her son is born, he is set adrift in a small boat, wrapped in rich cloths and identified on a small ivory tablet carved by his mother, telling only of his noble but sinful origin, not his name or place of birth.

Miraculously, the infant survives and is found by two fishing men from the imaginary village of St. Dunstan in a channel of the North Sea. He is adopted by the abbot Gregorius, who baptizes him and gives him his own name. The baby is reared by the wife of one of the fishermen. At the age of six, he moves to the monastery and becomes a scholar, studying Latin, grammar, theology, and law, all the while dreaming of becoming not a monk but a knight-errant. When at seventeen he learns of his origins by eavesdropping on his foster mother, he determines to leave the monastery, bearing the ivory tablet, in search of his parents.

Gregorius’ ship brings him to Bruges, where the duchess Sibylla lives like a nun, refusing to give herself in marriage to any man. The most persistent suitor, King Roger of Arelat, has for years been conducting a war between Burgundy and Flaundres-Artoys. Thus Gregorius, without planning it, becomes Sibylla’s knight, defeats King Roger, and marries the duchess.

Their life is blissful for three years. A daughter is born to them, named Harrad, and Duke Gregorius is known throughout the land for his mild and merciful judgment. Their happiness comes to an end when Sibylla’s maid, having spied on the duke, tells her mistress of the ivory tablet. Revealing her knowledge to the duke, Sibylla must once again be parted from her lover, who is not only her son but also her husband.

Gregorius, dressed as a beggar, departs immediately to do penance wherever God may lead him until death. Again, a fisherman and his wife are the source of a turning point in his life. The wife pities the beggar and suspects that he may be a saint, but her rough and cruel husband provides him with what he wants: a high rock to which he is fastened with fetters. The fisherman then hurls the key into the sea. For seventeen years the beggar survives, nourished by a milklike substance provided by the earth, the ancient mother of all, and he gradually turns into a small, prickly, hedgehog-like creature.

Gregorius’ life is again changed by war, this one caused by two factions in conflict over the choice of a pope; the two contenders die, and the Romans decide to leave the choice this time to God. A simultaneous dream vision is vouchsafed to two Romans, a wealthy man named Probus and a cardinal named Liberius. The two follow the instructions of the Lamb, who has appeared to them in their revelation, and...