It's 1962 New York and Barbara Novak (RENEE ZELLWEGER) is an author who's just written a female empowerment book by the name of "Down With Love." In it, she proclaims that women shouldn't confuse sex with love and should treat the former like a plaything, much like men do. Despite the efforts of her editor, Vikki Hiller (SARAH PAULSON), and the publishing power of head boss Theodore Banner (TONY RANDALL), however, the book barely gets noticed.
That is, until it's mentioned on the Ed Sullivan show. It's soon flying off the shelves, resulting in wives and other women suddenly refusing to act the way the men in their lives expect them to. That doesn't sit well with swinging playboy and ace reporter Catcher Block (EWAN McGREGOR). Ever the ladies man, he believes that all women want is love and marriage and thus sets out to prove Barbara to be a fraud.
Yet, since he's already stood her up several times, he takes on the personality of Zip Martin, a genial NASA astronaut who'd never dream of taking advantage of a woman and has never heard of Barbara or her book. Since she's become public enemy number one to most every man in the country, she instantly falls under his spell.
As his boss, Peter MacMannus (DAVID HYDE PIERCE), goes along with the ruse and finds himself drawn to Vikki, Catcher does what he must to make Barbara fall head over heels in love with him, all while trying to be careful that he doesn't fall for her.