Kit and David were college sweethearts. Now married and in their thirties, they live in Kit's childhood home in Bay Ridge, Brooklyn. While David has a successful career, jetting off on work trips to exciting destinations, Kit is stuck in a loop. She keeps quitting her job managing her sister's bakery to seek a more ambitious profession, but fear of failure always brings her back to Sweet Cheeks. Kit finds a fraught solace in cycling through fad diets, which David, in his efforts to be supportive, follows along with her. Their latest program is the Radiant Regimen, an intense cleanse, and Kit is optimistic about embarking on a new chapter of healthy eating and self-control.

Hungry in more ways than one, she soon falls into a flirtation with a carpenter named Matt who is building new shelves for the bakery kitchen. Unable to resist their mutual attraction, Kit and Matt soon begin a passionate affair. Kit suppresses her guilt by obsessing over her diet, pushing herself in greater extremes. Told in precise, intimate detail, Cheat Day is "an incredibly likable novel of hungers controlled and liberated, and marriage's gray areas" (Booklist) that explores monogamy versus monotony, deprivation versus indulgence, and limitations of modern wellness.