Sir Wilfrid Robarts (Charles Laughton), a master barrister in ill health, takes on Leonard Vole (Tyrone Power) as a client, despite the objections of his private nurse, Miss Plimsoll (Elsa Lanchester), who says the doctor warns him against taking on any criminal cases. Leonard is accused of murdering Mrs Emily French, a rich, older widow who had become enamoured of him, going so far as to make him the main beneficiary of her will. Strong circumstantial evidence points to Leonard as the killer, but Sir Wilfrid believes Vole is innocent.
When Sir Wilfrid speaks with Leonard’s German wife Christine (Marlene Dietrich), he finds her rather cold and self-possessed, but she does provide an alibi, although by no means an entirely convincing one. Therefore, he is greatly surprised when, at the end of the trial, she is summoned as a witness by the prosecuting barrister. While a wife can not be compelled to testify against her husband, Christine was in fact still married to a German man when she wed Leonard (who was in the Royal Air Force and part of the occupation forces in Germany). She testifies that Leonard admitted to her that he had killed Mrs French, and that her conscience forced her to finally tell the truth.
During the trial in the Old Bailey, Sir Wilfrid is contacted by a mysterious woman who, for a fee, provides him with letters written by Christine herself to a mysterious lover named Max. The affair revealed by this correspondence gives Christine such a strong motive to have lied that the jury finds Leonard not guilty.
However, Sir Wilfrid is troubled by the verdict. His instincts tell him that it was “…too neat, too tidy, and altogether too symmetrical!” His belief proves correct when Christine, left alone with him by chance in the courtroom, informs him that he had help in winning the case. Sir Wilfrid had told her before the trial that “…no jury would believe an alibi given by a loving wife”. So, she had instead given testimony implicating her husband, had then forged the letters to the non-existent Max, and had herself in disguise played the mysterious woman handing over the letters which then discredited her own testimony and led to the acquittal. She furthermore admits that she saved Leonard even though she knew he was guilty because she loves him.
Leonard has overheard Christine’s admission and, now protected by double jeopardy, cheerfully confirms to Sir Wilfred that he had indeed killed Mrs French. Sir Wilfrid is infuriated at being had by them both. Christine also suffers a major shock when she finds Leonard has met a younger woman and is now going abroad with her. In a jealous rage, Christine grabs a knife earlier used as evidence (and subtly highlighted by Sir Wilfrid’s monocle light-reflection), and stabs Leonard to death. After she is taken away by the police, Sir Wilfrid, urged on by Miss Plimsoll, declares that he will take on Christine’s defence.