Ann Hamilton (Katharine Hepburn) is the spinsterish daughter of Professor David "Dink" Hamilton, a well-respected chemist. When Dr. Hamilton introduces a business associate, Alan Garroway, engineer-designer of the Garroway Distance Controller, a device which reportedly helped win WWII, Ann and Garroway feel an instant attraction and before long are married.
The problem is that Alan moves in very high society and when he throws a big party at their home in Washington, D.C. right after the marriage, Ann is humiliated to find herself badly out of fashion with the other ladies. She turns to Alan to teach her taste and polish, which he happily does, taking her to dress shops, buying her the best and hiring a housekeeper. Everything seems idyllic until Mrs. Foster, an old neighbor of Alan's, makes mention of Michael, Alan's brother. Surprised that she's not heard of a brother before, Ann asks Alan about him and is surprised by Alan's explosive reaction. He informs Ann that Michael, who ran off during the war, was a manipulator who stole their mother's love and their family company's money.
Alan's obvious distress at anything to do with his brother convinces Ann that she must help him get over the past by being a loving wife, but that is not so easy as she discovers facts contrary to Alan's story. She finds a book of poetry belonging to Michael and meets Silvia Burton, a socialite who was in love with Michael. Silvia's portrayal of Michael doesn't match Alan's, and she shocks Ann when she voices her belief that Alan murdered his brother. In spite of herself, Ann becomes fascinated with Michael and secretly goes to the ranch that once belonged to him. There she finds an unexpected and peaceful beauty. She talks about it to the ranch's sympathetic caretaker. But as she enjoys the evening alone there, Alan shows up, enraged and accusing. He eventually tries to smooth things over, but Ann's doubts about his story and her fear that he may indeed be a killer consume her until Michael at last appears. Unfortunately, Michael's presence not only reveals the truth about Alan and himself, it ends up throwing Ann herself into a murder plot.