Kit and David were college sweethearts. Now married and twelve years older, they live in Kit's childhood home in Bay Ridge, Brooklyn. While David has a successful career, jetting off on work trips to exotic 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 right 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 seventy-five-day cleanse, and Kit is optimistic about embarking on a new chapter of clean eating and self-control.

But 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 fall into a passionate affair. Kit suppresses the guilt of her betrayal by adhering more and more strictly to the Radiant Regimen, pushing the diet, and her infidelity, to greater extremes.

Told in precise, intimate detail, Cheat Day is a sharply comic novel that explores family, loyalty, monogamy versus monotony, deprivation versus indulgence, and the limitations of modern wellness.