In London, Maddalena Anna Paradine 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. Paradines solicitor, Sir Simon Flaquer, hires Anthony Keane, 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. Keanes wife, Gay, notices his growing obsession, and although he offers to relinquish the case, presses him to continue. She knows that a guilty verdict, followed by Mrs. Paradines hanging, will mean that she will lose her husband emotionally forever. The only way that she can regain her husbands love and devotion is if he is able to obtain a not guilty verdict for Mrs. Paradine.

Meanwhile, Keane himself starts to focus his legal efforts on Colonel Paradines French-Canadian manservant, Andr Latour, DCM Bar. Keane sees Latour as a scapegoat on whom he can pin the crime of murder, but this strategy backfires. After Keane repeatedly badgers Latour in court, triggering an angry outburst, word comes that Latour has killed himself.

Mrs. Paradine is coldly furious that (contrary to her express instructions) Keane has destroyed Latour, who was her lover. In the witness box, she tells Keane that she hates him and that he has murdered the only person she loved. She goes so far as to say that she poisoned her husband in order to be with Latour.

Keane is overwhelmed. Attempting to summarize, he improvises a brief and faltering speech, admitting how poorly he has handled the case; unable to continue speaking, he has to leave the court. He stays overnight at Sir Simons office, knowing that his career is in ruins. Finding him there, his wife offers reconciliation and hope for the future.