One of Bob's colleagues has just written an article about Dr. Helen Gurley Brown Natalie Wood , a young psychologist and author of the best-selling book Sex and the Single Girl, a self-help guide with advice to single women on how to deal with men. The article raises doubts on her experience with sex and relationships. Helen is very offended, having lost six appointments with patients due to the article discrediting her as a "23 year-old virgin. Bob wants to follow up by interviewing her, but she refuses.
Bob's friend and neighbor, stocking manufacturer Frank Broderick Henry Fonda , is having marriage issues with his strong-willed wife Sylvia Lauren Bacall , but can't find the time to go to a counselor. Therefore, Bob decides to impersonate Frank and go to Helen as a patient, with the goal of getting close to her in order to gather more information. In exchange, he'll report back to Frank her advice. During their first couple of sessions, Bob acts shy and smitten, and tries to gently seduce Helen.
She seems to respond to Bob's courteous advances, all while insisting it's a transfer and that she'll play the role of Sylvia to the benefit of his therapy. After he fakes a suicide attempt, the two of them end up making out in her apartment, with Bob realizing he's actually falling for Helen, which is the reason he still has not written anything about her, prompting an ultimatum from his boss.
Helen panics at the idea that she's in turn falling for a married man, and upon suggestion from her mother, she meets Sylvia and tells her to go back to work at Frank's office, where the two of them first met and could stand together against Frank's business rivals. Sylvia had initially rejected that suggestion coming from Frank who had heard it from Bob , but she ultimately decides to follow the advice, thus reconciling with her husband.
A terminally lovestruck Bob forces another meeting with Helen and tries to convince her his marriage isn't legal, but Helen insists on hearing it from his wife and secretly asks her to come to her office. In the meantime, Bob asks his girlfriend, night club singer Gretchen Fran Jeffries , to pose as his wife or better, Frank Broderick's wife , and when she cancels at the last minute because of an audition, he asks his secretary Susan Leslie Parrish to go instead.
Without telling him, Gretchen decides to forgo her audition, so she also shows up at Helen's office. Witnessing three different women claiming to be Mrs. Broderick, Helen gets extremely confused, while a furious Sylvia calls the police on Frank, who is then arrested for bigamy.
Helen comes visit Sylvia with fellow psychiatrist Rudy DeMeyer Mel Ferrer , who has had a crush ever since the article intimated she might be a virgin. Shocked, she asks Rudy to take her to Fiji. In the meantime, Bob refuses to let the magazine publish anything about Helen, and is consequently fired. In a frantic final act, Helen and Rudy are driving to the airport, while Frank, after being released, didn't understand Sylvia learned the truth so has decided to quit everything and elope to Hawaii with Gretchen; at the same time, Bob is chasing after Helen, and Sylvia is chasing after Frank.
During the chase, while constantly changing places on the two cars and two cabs involved, and while eluding a zealous cop's attempts at stopping them, the three couples clarify their feelings for each other, and at the airport, Frank and Sylvia reconcile and depart for Fiji, Rudy and Gretchen console themselves with a trip together to Hawaii, and Helen forgives Bob, who has already found a new job as a respectable journalist, and the two of them fly to Vegas to marry.
Rudy even ending up with Gretchen.