Joanna Eberhart (Katharine Ross) is a young wife who moves with her husband Walter (Peter Masterson) and two children from New York City to the idyllic Connecticut suburb of Stepford. Loneliness quickly sets in as Joanna, a mildly rebellious aspiring photographer, finds the women in town all look great and are obsessed with housework, but have few intellectual interests. The men all belong to the clubbish Stepford Men's Association, which Walter joins to Joanna's dismay. Neighbor Carol Van Sant's (Nanette Newman) sexually submissive behavior to her husband Ted, and her odd, repetitive behavior after a car accident also strike Joanna as unusual.
Things start to look up when she makes friends with another newcomer to town, sloppy, irrepressible Bobbie Markowe (Paula Prentiss). Along with glossy trophy wife Charmaine Wimperis (Tina Louise), they organize a Women's Lib consciousness raising session, but the meeting is a failure when the other wives hijack the meeting with cleaning concerns. Joanna is also unimpressed by the boorish Men's Club members, including intimidating president Dale "Diz" Coba (Patrick O'Neal); stealthily, they collect information on Joanna including her picture, her voice, and other personal details. When Charmaine turns overnight from a languid, self-concerned tennis fan into an industrious, devoted wife, Joanna and Bobbie start investigating, with ever-increasing concern, the reason behind the submissive and bland behavior of the other wives, especially when they learn they were once quite supportive of liberal social policies.
Spooked, Bobbie and Joanna start house hunting in other towns, and later, Joanna wins a prestigious contract with a photo gallery with some photographs of their respective children. When she excitedly tells Bobbie her good news, Joanna is shocked to find her freewheeling and liberal friend has abruptly changed into another clean, conservative housewife, with no intention of moving from town.
Joanna panics and at the insistence of Walter, visits a psychiatrist where she voices her belief that all the men in the town are behind a conspiracy of somehow changing the women. The psychiatrist recommends she leave town until she feels safe, but when Joanna returns home, the children are missing. The marriage devolves into domestic violence when Joanna and Walter get in to a physical scuffle. In an attempt to find her children, she hypothesizes Bobbie may be caring for them. Joanna, still mystified by Bobbie's behavior, is desperate to prove her humanity but intuitively stabs Bobbie with a kitchen knife. But Bobbie doesn't bleed or suffer, instead going into a loop of odd mechanical behavior, thus revealing she is a robot.
Despite feeling she may be the next victim, Joanna sneaks into the mansion which houses the Men's Association to find her children. There, she finds the mastermind of the whole operation, Dale "Diz" Coba, and eventually her own robot-duplicate. Joanna is shocked into paralysis when she witnesses its soulless, black, empty eyes. It is then suggested that the Joanna-duplicate strangles the real Joanna. In the final scene, the duplicate is seen placidly purchasing groceries at the local supermarket, along with the other "wives" all wearing similar long dresses, large hats and saying little more than hello to each other. The final shot focuses on Joanna's now-finished eyes.