New in New York City, Barbara Novak arrives at Banner House to present her new work Down with Love, a book the intent of which is to free women from love, teach them to use sex without love, and to replace the need for a man with things such as chocolate. Following her rules would, she believes, help to give women a boost in the workplace. However, the men in Banner House refuse to support the book. The only way Vicky Hiller, Barbara's editor, finds to promote her book is for Barbara to meet Catcher Block - a successful writer and notorious ladies' man - but he avoids her repeatedly by postponing their dates until she gets fed up, insults him and walks out.

Vicky and Peter McMannus, Catcher's boss and best friend, take a liking to one another. However, their relationship revolves around Barbara and Catcher, and neither is brave enough to express their feelings for the other. Peter feels overshadowed by Catcher's strong personality, and Vicky wants to see strength in her lover.

Barbara starts promoting her book with Vicky's help, and things take off when they get Judy Garland to sing the song "Down with Love" as a promotion to the book on The Ed Sullivan Show. Sales sky-rocket, as housewives and women around the world buy the book and rebel against their men; Catcher now wants to meet Barbara, but now it is she who rejects him.

It all comes to a boiling point when Barbara appears in a late night show talking about a chapter from the book - "The Worst Kind of Man" - and cites Catcher Block on national television as the perfect example. His date rejects him, which infuriates him, and he swears he will prove Barbara is the same as every other woman, wanting the same things men do.

He arranged for a casual meeting in a laundry shop, taking advantage of the fact that she has never met or seen him, and he poses as an astronaut, Major Zip Martin. Barbara is immediately infatuated with this man as he appears to have no idea who she is - men avoid her, viewing her as the enemy since the publication of her book. He takes her to the most fashionable locations in New York while maintaining considerable sexual tension between them by feigning naivete and a desire to remain chaste until he is "ready" for a physical relationship. But he starts falling for her, and it gets harder to go through with his plan.

When Barbara finds Catcher/Zip at a party he is almost caught out, and decides it is time to take everything to the next level: he tells Barbara that Catcher Block wants to interview him for an exposé on the NASA space program and asks her to accompany him. It is his own apartment and he sets everything up to record her saying she loves him. But then it she who reveals the truth: she knew he was really Catcher from the beginning, but she also lied as she is not Barbara Novak but Nancy Brown, one of Catcher's many secretaries, who fell in love with him whilst working at Know, but turned him down when he asked her out because she did not want to be just another one in his long list of romances.

She tells him she did this to be different from all the women he knew, and make him love her. They realize that he loves her, but as he is proposing, one of Catcher's many lovers appears and thanks Barbara for what she's done for womankind. Barbara realized that she does not want love or him as she has become a real 'down with love' girl. It is also there where Vicky and Peter's relationship changes when she insults him for helping Catcher. Peter realizes he is indeed like any other man and takes Vicky to Catcher's apartment to take things to the next level.

Days later, Catcher is completely depressed; all his efforts to win Barbara back have failed. Even his exposé is ruined now that Barbara has told her story in her own magazine. Peter is also depressed as his relationship with Vicky is now apparently based only on sex. Catcher realizes he can do something and writes a new exposé "How Falling In Love With Barbara Novak Made Me A New Man". He learns there is an opening at Barbara's magazine and goes for an interview with her. There, he tells her how much she changed him, and it is obvious she wants him but turns him down anyway; he says he wished there could be a middle ground for them "somewhere between a blonde and a brunette", referring to her real persona, where she was a brunette.

As he is leaving her office, he realized she is not coming after him, but surprises him on the elevator, showing him a bright red hair style, meaning she has found the middle ground and she wants to be with him. They fly to Vegas to get married which influences Vicky and Peter, who also decide to get married.

The end credits show their marriage has resulted in a new book intended to end the battle of the sexes. The pair end by singing "Here's To Love".