One hot summer in Manhattan, book editor Richard Sherman escorts his wife Helen and son Ricky to the train station, from which they and numerous other families are leaving to escape the city's heat. After agreeing to Helen’s admonitions not to smoke or drink, Richard briefly joins the other “summer bachelors” in ogling a pretty woman, but firmly tells himself that he will not be like other husbands who run amok while their families are away. Richard returns to his office at Brady & Co., where his unusually vivid imagination helps in the designing the company’s lurid covers of paperbacks. After a bland, healthy dinner, Richard goes home and is about to work on a new manuscript, Of Man and the Unconscious by Dr. Ludwig Brubaker, when he is interrupted by the outside door buzzer. A stunning blonde enters and tells Richard that she is his new neighbor, as she is renting the apartment above his for the summer, and the awestruck Richard’s neck cracks alarmingly as he cranes to watch her ascend the stairs. Determined to enjoy a quiet evening, Richard resolves not to think about The Girl, whose name he did not learn, and returns to Brubaker’s book. Before long, however, Richard begins pondering Helen’s intention to call him at 10:00 and decides that she must not trust him, even though he has been faithful during their seven years of marriage. As Richard debates the matter, he imagines Helen sitting opposite him on the patio and hears her laugh when he states that he is attractive to other women. The vision of Helen continues to chuckle as Richard dramatically spins a tall tale about being romantically accosted by his secretary, Miss Morris; a nurse; and Helen’s own best friend, Elaine. Helen chides Richard for imagining things in CinemaScope with stereophonic sound, then disappears, but before Richard can return to his manuscript, the real Helen telephones early. Richard is disconcerted to hear that she ran into Tom MacKenzie, a writer of lurid romances, on the train to Maine, yet promises that he is following her advice. After hanging up, Richard gets up just before a huge tomato plant from the upstairs balcony crashes into his chair. The Girl, who was watering the plants, apologizes for knocking over the plant, and Richard invites her for a drink. Nonplussed by the Girl’s announcement that she leaves her underwear in the refrigerator to keep cool, Richard frantically prepares for her visit. While searching for mood music, Richard selects Rachmaninoff’s Piano Concerto No. 2 and imagines himself seducing the Girl with his magnificent playing of the powerful music. When the doorbell rings, Richard comes out of his reverie and instead finds Krahulik, the building’s janitor who has come to take the rugs. Richard quickly dismisses Krahulik, then welcomes the Girl, a bubbly, naïve model who appears in television commercials. The Girl laments the fact that her apartment does not have air conditioning, as the Shermans’ does, and relates that she attempted to sleep in a bathtub full of cool water, but had to call a plumber after her toe got stuck in the faucet while she was trying to stop a leak. When the Girl notices his wedding ring, Richard admits that he is married, but she responds that she is pleased he is married, as nothing “drastic” can happen. Richard commiserates with her complaint that men are continually asking her to marry them, then plays “Chopsticks” with her on the piano. Their duet ends when Richard, imitating the earlier seduction scenario he had imagined, attempts to kiss the Girl and they fall off the piano bench. Bewildered but not unhappy, the Girl exits gracefully when the nervous Richard asks her to leave. The next morning, worried that he is becoming a dissolute lecher, Richard asks his boss, Brady, for two weeks off to spend with Helen but Brady refuses. Richard then reads Brubaker’s theory about “the seven year itch,” which posits that most married men commit adultery during the seventh year of marriage, and that the statistic grows higher during warm weather. When Brubaker arrives to discuss his book, Richard confesses that he attempted to “terrorize” a young lady the previous evening. Intrigued by Richard’s twitching thumb, Brubaker listens and advises him to give himself more room than a piano bench if he feels the impulse to “terrorize” again. After Brubaker leaves, Richard imagines the Girl telling the plumber about his attack upon her, and the news then spreading throughout New York until Ricky and Helen see it on television in Maine. In despair, Richard decides to telephone Helen to learn if she has discovered his indiscretion. Instead of Helen, the phone is answered by a babysitter, who informs Richard that Helen is on a hayride with Tom. At first Richard is pleased, believing that Helen must not suspect anything is wrong if she is out having fun, but then begins to imagine that Tom has arranged a private hayride, with no other passengers, no driver and blinkered horses, in order to make a pass at the receptive Helen. Infuriated with Helen’s supposed behavior, Richard takes the Girl to dinner and a movie, and after the film, admires her legs as she stands over a subway grating and her skirt is blown up by the air from passing trains. Richard and the Girl return to his apartment, which is cool from the air conditioning, and while Richard prattles about psychology and the possibility that the Girl subconsciously loves him, she worries about the heat. Working up her courage, the Girl asks him if she can spend the night, and after determining that she intends to sleep in the living room chair, a nervous Richard assents. Their conversation is interrupted by Krahulik, who has come again to collect the rugs. Krahulik spots the Girl, who is hiding in the chair, but Richard bluffs through the situation by stating that she came to retrieve her tomato plant. Richard is forced to usher the Girl out, along with Krahulik and the plant, but is surprised soon after when she appears through the ceiling via the trap door separating the stairs between their apartments. The Girl happily announces that they can go back and forth all summer without anyone knowing, and Richard, horrified by so much temptation, spends a troubled night on the couch. In the morning, while waiting for the Girl to awaken, Richard imagines that Helen, informed by Krahulik about his rendezvous with the Girl, has returned from Maine to shoot him. The Girl calms Richard, and when he tells her that Helen is never jealous of him because no pretty woman would want someone like him, the Girl informs him that women always want a shy, sweet man rather than some egotistical, handsome lout. Giving him a tender kiss, the Girl assures him that she would be jealous of him if she were his wife, then goes into the kitchen. Richard is then surprised by the arrival of Tom, who has come to pick up Ricky’s kayak paddle. Richard assumes that Tom has come to ask for a divorce on Helen’s behalf, however, while Tom is baffled by Richard’s talk of having a blonde who might be Marilyn Monroe in the kitchen. Determined to be with his family, Richard slugs Tom and knocks him out, then tells the Girl that she can use the apartment while he is gone. Richard grabs the paddle and runs out into the street before being stopped by the Girl, who tosses him his shoes from the window. Richard’s neck cracks again after one last look at the Girl, then he waves goodbye as he hurries to the train station.