Returning to London after extensive travels abroad, Mary and husband Sherlock Holmes receive an unexpected visit from Sherlock's estranged son Damian. Years ago, Damian was arrested following the murder of his drug dealer. No more people have died, and Damian is once again a suspect. Further complicating matters, Damian's wife and child are missing. While Sherlock helps Damian search for his loved ones, Mary launches her own investigation in an attempt to reveal Damian's troubled history.