Mandie, Celia, Mrs. Taft and Senator Morton arrive safely in London and take Jonathan, the young stowaway from Mandie and the Shipboard Mystery, along with them.

A telegram from an unknown source is waiting for Mandie and an unexplained message has been left at their hotel. Who is so interested in what Mandie is doing? And why don't they sign their names? Is this a new adventure-or danger?

When they decide to deliver Jonathan to his aunt in Paris, adventure begins. But what at first seemed like a simple mystery turns into a dangerous trap and an encounter with the disappearing stranger.

Mandie's in Paris and so are the spies!