In 1974, a bookish, overweight, depressed teenage girl named Gina vacations for two weeks at the home of a client of her seamstress mother's. The woman, Marnie, is beautiful with four beautiful and confident (but not intellectual) children: Becky, Josh, Tom, and Gabriel.

She is visiting in part because Marnie has offered to take her to Wing Lodge, the historic home of Gina's favorite novelist, John Morrison. Marnie compliments Gina's intellectualism and Gina thinks of all the attributes Marnie's children have that she doesn't. Gina is disappointed by Wing Lodge--it seems more expensively furnished than she would've expected, like a place where she is an unfamiliar stranger even though she knows Morrison's books so intimately.

Often during her stay the family will go on outings to the beach and Marnie will stay at home, pretending to be studying but in actuality using her time alone in the house to examine all the rooms, eat bits of food, and help herself to cigarettes and alcohol. One such day, she realizes Tom is still home, cutting grass behind the house. She hides in her room for the remainder of the time, worried what he, whom she has had sexual fantasies about, thinks of her.

Another day, she and Josh are left at home alone. Josh is building towers out of cards, and she offers to show him a magic trick. She pretends to give him the power to blindly sort the cards into red and black. He's amazed by the magic trick, even though she thinks it's simple and obvious.

She imagines having sex with him, but the moment passes. They play games together for the rest of the afternoon and she feels what she figures is happiness.

25 years later, Gina, now a literary scholar, visits Wing Lodge again. She is divorced and her new lover wants to move in with her. She is moved to tears by a passage she sees in one of the historic drafts of Morrison's work about "a scene of a woman putting her happiness into a man's hands" even though she thinks it "ought to disgust her." She remembers to card trick with Josh.