Two years after "The Mystery of the Yellow Room" that shook the public opinion of France, the astute journalist Joseph Rouletabille is searching for clues about his past.
At the same time, Mathilde Stangerson marries her beloved Robert Darzac, but during the ceremony, an indistinct threat looms in the atmosphere. Although no one admits it, everyone expects that the now-deceased Larsan will appear to threaten their lives. However, everything goes smoothly, and the happy couple leaves for their honeymoon.

A few days after a telegram signed by Mathilde Stangerson unsettles Rouletabille. It contains only two words: "Save us".
The developments are rapid, and the couple, along with their friends, ends up in a well-fortified medieval castle on the French Riviera, where it is absolutely impossible for anyone to break in and harm Mathilde Stangerson. And yet… The unthinkable happens in the most mysterious way. Rouletabille is now called upon not only to reverse a crime but also to face a dreadful truth head-on.

Gaston Leroux, after 'The Mystery of the Yellow Room,' offers the reader of detective books an even more suspenseful and extreme puzzle. Can you solve it?