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Based upon the book The Four Marys, by Fanny Heaslip Lea, Man-Proof involves members of the smart and sophisticated set of Manhattan, circa 1938. Mimi Swift (Myrna Loy), the daughter of a successful romance novelist, is having no luck in getting the man she loves. Although she has made her interest very clear to Alan Wythe (Walter Pidgeon), he is more concerned with marrying someone who can support him. Still, Mimi is unprepared for the shock of discovering that Alan is engaged to her wealthy friend Elizabeth Kent (Rosalind Russell) -- and even more unprepared when they want her to suffer through the ceremony firsthand, as a bridesmaid. Ever the lady, Mimi forces herself to attend the wedding and attempts to put up a good front; unfortunately, she takes a little too much advantage of the free flowing champagne, with the result that she lets Alan know that she hasn't given up on him. While Alan and Elizabeth are on their honeymoon, Mimi becomes involved with Jimmy Kilmartin (Franchot Tone), an illustrator at the New York Chronicle. After the honeymooners return, Mimi tries to accept the fact that lan is Elizabeth's, but she just can't and lets Elizabeth know that things will never be the same. For his part, Alan seems to want to have his cake and eat it, too, and indicates to Mimi that perhaps something can be worked out. As they are talking, Elizabeth enters, puts two-and-two together and tells Alan she knows he only married her for her money and suggests they get a divorce. Eventually, Elizabeth takes Alan back and Mimi suddenly realizes that there is more to Jimmy than she had previously observed -- and more than enough to make her happily forget about Alan.