I'M OFTEN WARY of relationship novels. In the wrong hands, they can make the reader feel like the single roommate of a tumultuous, loudly cohabitating couple. But Lauren Groff, author of 2012's beautifully written Arcadia, delivers a gripping and fascinating take on the subject in Fates and Furies, her latest novel.

The first half is the point of view of the magnetic failed-actor-turned-playwright Lotto, focusing as much on his eccentric upbringing and career as his love and marriage to his wife, Mathilde. Rich with captivating backstory and excerpts from Lotto's plays, this part of the novel is charming, but I was bothered by how flat Mathilde's character was, often mere accessory to her husband. For instance, we are frequently reminded that Lotto wishes he and Mathilde had children, but we are not told why they do not. Does Lotto know his wife's feelings on the matter? Does Lotto know his wife's feelings at all? Why, in a novel about only two characters, are we given so little access to Mathilde? Such questions begin to accumulate, and the omissions become troubling—and at odds with other moments, where Groff writes sensitively and virtuosically about her characters' relationship, about the ways their bond is strengthened and strained over the course of their lives together.

The story relies on several twists and revelations, and it's difficult to discuss these without giving away some of Groff's thrilling surprises. In the book's second half, told from Mathilde's point of view, the novel's omissions begin to make sense, and contribute to an increasingly captivating plot. Villains emerge, and they are not who we expect. Interesting thematic complications arise, too. Ultimately, the novel asks how well two people can know one another. To what extent is the beloved a creation of the lover, a projection of what one imagines they need to complete themselves?

Fates and Furies is not perfect, but its structural shortcoming is also its brilliance: Neither half works without the other, and together, they make a stunning whole.