This is one of those early romantic movies Clark Gable and Joan Crawford used to do.
Diane Lovering (Crawford) is the unapologetic mistress of a New York shipping magnet, Richard Field (Otto Kruger). Richard would love to divorce his wife Louise (Marjorie Gateson) and marry Diane, but Louise likes her lifestyle too much the way it is, and threatens to take Richard's two sons away where he would never see them again.
Richard feels guilty about this and buys Diane an ocean cruise to Argentina to ease her pain and to give her time to think about whether she wants to stay in this arrangement the way it is or not. On the ship she meets Mike Bradley (Gable) who has a South American cattle ranch.
Naturally, they fall madly in love with each other, but Diane's loyalty to Richard keeps her from sealing the deal until she talks with Richard first. When she arrives back in New York, she finds out that Richard is divorcing Louise, despite never seeing his kids again. Diane resigns herself to marrying Richard and sends Mike a quick Dear John letter.
Later, Diane bumps into Mike while shopping downtown and discovers that she still loves Mike more than Richard.
As for the plot, Diane is "fond of" her married boss, who wants to marry her, but his wife won't give him a divorce. Diane is fine with that, if that's how it has to be, but the Boss sends her on a cruise to think it over. Clark Gable is on the cruise--whoopsie! Guess who she ends up with?
Bonus: Una O'Connor, whom it's always a pleasure to see.