In London, Maddalena Anna Paradine (Alida Valli) is a very beautiful and enigmatic young Italian woman who is accused of poisoning her older, blind husband, a wealthy retired colonel. It is not clear whether she is a grateful and devoted wife who has been falsely charged or a calculating and ruthless femme fatale.
Mrs. Paradine's solicitor, Sir Simon Flaquer (Charles Coburn), hires Anthony Keane (Gregory Peck), a brilliant and successful barrister, to defend her in court. Although Keane has been happily married for eleven years, he instantly becomes deeply infatuated with his client. Keane's wife, Gay (Ann Todd), sees his obsession, and although he offers to relinquish the case, presses him to continue. She knows that a "guilty" verdict, followed by Mrs. Paradine's hanging, will mean that she will lose her husband emotionally forever. The only way that she can regain her husband's love and devotion is if he is able to obtain a "not guilty" verdict for Mrs. Paradine.