September, 1925. Mary Russell and Sherlock Holmes are looking forward to spending time with Holmes' son, the famous artist Damian Adler, and his family in the French countryside. But on their arrival, they discover that the Adlers have fled from a mysterious threat.

In the ominously empty house, Russell discovers clues including an old journal written in a nearly impenetrable code. As she delves deeper, it appears that the secrets of the past are reaching into the present. Could there be things about Holmes' own history that even the master detective does not perceive?