When Leonard Vole is arrested for the sensational murder of a rich, middle-aged widow, the famous Sir Wilfrid Robarts agrees to appear on his behalf. Vole’s only alibi witness is his wife, Christine (Marlene Dietrich), a German actress. Christine confirms her husband’s alibi but implies that he had asked her to lie and has not been truthful about his relationship with Mrs French, having ‘a way with women’ She promises to be very convincing on the witness stand, even if she is lying. Despite this Sir Wilfrid, believing in Leonard's innocence, takes the case.
Christine is then called as a witness for the prosecution and testifies that Leonard told her: ‘I killed her.’
That evening Sir Wilfrid eceives a phone call from a Cockney woman who says she has ‘the goods’ on Christine and asks Sir Wilfrid meet her at Euston Station. The woman snarls her hatred of Christine and for £40 hands over a packet of ‘juicy’ letters from Christine to a man named Max, who she says had been her lover before he fell in love with Christine. She disappears after showing Sir Wilfrid a scar on her face, which she said came from Max.
The next day in court Sir Wilfrid confronts Christine with a letter stating her intention to blame Leonard for the murder so that she could be free to be with Max. When Sir Wilfrid tricks her into admitting that she wrote the letter the jury quickly returns a not guilty verdict but Sir Wilfrid worries that everything was ‘too neat’. He warns Christine that she will go to jail for perjury, but she says that the testimony she gave was the truth; she assumes the Cockney woman’s accent and reveals that Max and the letters were figments of her imagination. She had to tell the truth about Leonard and then discredit her own testimony because the jury never would have believed uncollaborated supportive testimony from a loving wife.
When Leonard tells Christine he is leaving her for a younger woman she stabs him with the murder weapon which is still on a table in the courtroom.
Ray Sebastian, Harry Ray, & Gustaf Norin were credited for makeup; according to the IMDb Wally Westmore & Charles Gemora were responsible for Marlene Dietrich’s makeup.