"Girl in 313" is a B-movie from Twentieth Century-Fox. You can tell it's a B because it is less than an hour in length and stars lesser known actors. What is a B? Well, it's a cheaper, more quickly made film designed as a second film for a double-feature...which was the norm back in the day. Yes, you'd pay your admission and get to see two different films...plus, likely, you'd get a cartoon, newsreel and some other sort of short!
A valuable brooch is stolen during a fashion show. An agent working for the store vows to retrieve the brooch and the trail leads to a pretty young lady thief. Soon, however, the pair join forces....which is ridiculously improbable. What happens next...even more improbable. I won't say more as I don't want to spoil the suspense.
Like many B-movies, the writing for "Girl in 313" sure isn't very good. In fact, it's poor for two main reasons. The big twist is unexpected...but also ridiculously improbable and hard to believe. Additionally, the use of a kooky character (the maid, Mary Treen) is most unwanted as it's supposed to be a serious mystery. Overall, pretty much what you'd expect in a cheap B....and nothing more.