Sigrid Undset, who won the Nobel Prize in literature in 1928, is best known for “Kristin Lavransdatter” (1920-1922), her trilogy of novels set in 14th-century Norway. Its English translation of the 1920s was a faux-medieval concoction, all gussied up with an abundance of ’tis, ’twould, ’twill, and methinks. And so it stood until the 1990s when Tiina Nunnally retranslated the entire trilogy, restoring the clarity and briskness of Undset’s actual prose style. She now performs the same service for the author’s lesser known work, a tetralogy, the first volume of which “Olav Audunssøn: I. Vows” has just appeared.

Set in 13th-century Norway, it is the story of Olav Audunssøn, whose widowed, dying father committed him, at 7, to his friend, Steinfinn Torressøn. Expansive with drink and fine feeling, Steinfinn adopts the boy as his foster son and pledges his own young daughter, Ingunn, to him in marriage.

Despite the slapdash nature of their betrothal, the children grow up understanding that they would eventually marry. Meanwhile, Steinfinn, lackadaisical about managing Olav’s inheritance and his own estate, is obsessed with avenging an offense against his honor. But his revenge, when it comes, ends up costing him his life. Now on his deathbed, he refuses to grant permission for 16-year-old Olav to marry Ingunn and she becomes a pawn in the plans of her father’s illegitimate half-brother, Kolbein, who has become her guardian.

The two young people find themselves trapped in a net of Christian moral strictures, ancient Norse codes of honor, and familial interest: They are bound together by promises and deeds, but not free to marry. It is an increasingly convoluted and unforgiving state of affairs amid which the character of each is tried and altered.

Olav has grown into a thoughtful if melancholy youth, but he kills a man in a fit of violent rage, an act which drives him into exile and out of the story’s main focus — though never out of Ingunn’s thoughts. Originally a capricious little miss governed by whim and impulse, Ingunn is determined to wait for Olav’s return, but fate, merciless and relentless, throws up temptation before which she, never a model of self-control, is finally helpless. She falls into acts that are not willed so much as succumbed to, only to be followed by remorse, shame and misfortune.

The plot, more of which I will not reveal, emerges naturally, intricately and inevitably from the makeup of the characters’ souls, especially Ingunn’s, which deepens throughout. As she did so powerfully in “Kristin Lavransdatter,” Undset matches the precision and force of her characters’ inner lives — lacerated by indecision, sunk in sorrow or transported by joy — with her evocation of a vanished age and depictions of the life-affirming beauty of nature.

This is a novel you wish would never end — and it doesn’t, not yet: The following volumes will be appearing over the next three years.

 Katherine A. Powers, a Minnesota native, also reviews for the Washington Post and the Wall Street Journal.

 Olav Audunsson: Vows

By: Sigrid Undset, translated from the Norwegian by Tiina Nunnally.

Publisher: University of Minnesota Press, 376 pages, $17.95.