Della and Perry's dinner at a fancy French restaurant is interrupted by a phone call and an envelope containing $500.00 from a mysterious woman named Marian Fargo. She wants him to make an exchange for her: the money for some documents regarding her fugitive brother. Unbeknownst to Marion, her own husband is in on the blackmail scheme which is being masterminded by one Samuel Carlin. Marion's husband and Carlin both turn up dead but Marion, who has fled on a bus, has an eyewitness, Diana Maynard, who can provide her with an alibi for the murders. Unfortunately for all concerned, Ms. Maynard has only one good eye and may not be telling the truth about who she is.
