September, 1925. After their recent adventures in Transylvania, Mary Russell and Sherlock Holmes look forward to spending time with Holmes' son, the famous artist Damian Adler, and his family in the French countryside. But when they arrive at Damian's house, they discover that the Adlers have fled from a mysterious threat.

In the ominously empty house, Russell discovers several crates packed with memorabilia related to the artist Horace Vernet, including an old journal written in a nearly impenetrable code. Intrigued, Russell sets about deciphering the intricate cryptograph.

The secrets of the past appear to be reaching into the present. Could there be things about Holmes' own history that even the master detective does not perceive?