Witness for the ProsecutionAirs Saturday, May 11 at 9:00 PM 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. Vole is accused of murdering Mrs. Emily French (Norma Varden), a rich, older widow who had become enamored of him, going so far as to make him the main beneficiary of her will. Strong circumstantial evidence all points to Vole as the killer.
When Sir Wilfrid speaks with Vole's German wife Christine (Marlene Dietrich), he finds her rather cold and self-possessed, but she does provide an alibi. Therefore, he is greatly surprised when she is called as a witness for the prosecution. While a wife cannot testify against her husband, it is shown that Christine was in fact still married to another man when she wed Leonard (although Vole, believing in good faith that he was married to Christine at the time, might still have qualified under the spousal privilege rule). 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, carefully recreated by art director Alexandre Trauner), 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!". And so it proves. By chance, he and Christine are left alone in the courtroom. She takes the opportunity to take credit for the whole thing. When she heard him say at the beginning that a wife's testimony would not be convincing, she decided to set it up so that hers would be given for the prosecution and then be discredited. An ex-actress, she had played the part of the mystery woman so well that Sir Wilfrid did not recognize her when he negotiated for the letters. She knew that Leonard was guilty; her testimony was the truth. Her letters are a fraud - Max never existed. When asked why she did it, she confesses that she loves Leonard.
Leonard appears and, now protected by double jeopardy, nonchalantly confirms what Christine had said. A young woman (Ruta Lee) then rushes into his arms. When he admits that he and the young woman are going away together, Christine kills him with a knife in a fit of fury. Sir Wilfrid remarks that Christine did not murder Leonard, but that she "executed him". Miss Plimsoll then cancels Sir Wilfrid's holiday, realizing that he cannot resist taking charge of Christine's defense.