The Mysterious Mannequin is a mystery that involves a Turkish rug, a hidden message, a mysterious mannequin and a client of Carson Drew who vanished some time ago.

The story begins with Carson Drew receiving a strange parcel in the mail that contains a Turkish rug. He suspects a link between the rug and his client Farouk Tahmasp, who disappeared before his case went to trial. Carson Drew asks Nancy to help him figure out the meaning of this mysterious gift.

Nancy thinks that the rug might contain a hidden message and she is proved right. A message for Carson is woven into the decoration surrounding the border of the rug. It asks Carson Drew to find a mannequin that belonged to Farouk.

Nancy decides that this must be the mannequin that Farouk had in the window display of his shop. Now, Nancy and her friends have to track down the mannequin in the shop. The problem is that the shop was closed two years ago. However, Nancy is not the type that lets little challenges like this stop her.

As Nancy and her friends go on the trail of the missing mannequin, the mystery leads them to strange places and many dangers, before they manage to unravel the puzzle.