Pickup on South Street (1953)

While on the subway to deliver a piece of film she believes contains corporate secrets, Candy's wallet is stolen by Skip McCoy. The film actually contains government secrets on their way to "the Commies", and Candy has been under surveillance by government agents. They see the theft, but Skip eludes them. Anxious to recover the film, Joey, Candy's ex-lover and the man using her as courier, convinces her to find the thief. She tracks Skip down through street connections while the police find him shortly after. Skip finally realizes the film is worth a lot more than he thought worth some people's life, in fact.