Age of Indiscretion.

A book publisher finds his business floundering, which prompts his socially ambitious wife to desert him for a society millionaire, leaving him with their young son. The publisher's fortunes improve dramatically, however, when a best-selling romance novelist decides to publish her new book with his firm. In the meantime, his ex-wife has married the millionaire, and she and her new mother-in-law come up with a plan to sue her ex-husband for custody of the boy.