A young woman is found attacked and bleeding, while locked in an impenetrable room: door locked, windows barred. Who attacked her and how? Only the brilliant Rouletabille can solve the crime!

I hadn't realized this, but it turns out the author of The Phantom of the Opera, Gaston Leroux, was the Arthur Conan Doyle of France, writing a bajillion mysteries and other pulp novels. This is very readable. I had the mystery from early on--and then got completely talked out of my theory, spending the rest of the book so far off track that I was in another genre. A fun read.