While out walking on the Sussex Downs, teenage orphan Mary Russell meets her neighbor Sherlock Holmes who has retired there to devote much of his time to studying bees.

When Mary Russell gains her independence at the age of 21, she goes off to London to experience her liberty. There she meets a charismatic woman preacher whose inner circle of disciples are dying in accidents too numerous to be accidental.

Shortly before being run down by a car in London, a lady archaeologist gives Mary Russell a mysterious box containing an ancient scroll that appears to be a letter written by Mary Magdalene.

Mary Russell joins Sherlock Holmes who is staying with the aging writer Sabine Baring-Gould in Dartmoor. In the neighborhood, a suspcious gold prospector from America has bought Baskerville Hall. Meanwhile mysterious events involving a giant hound of local legend have been occurring on the moor.

This novel goes back in time to the period skimmed over in The Beekeeper's Apprentice when Mary Russell and Sherlock Holmes leave England to escape their unknown persecutor. Sent by Mycroft to Palestine, which has recently come under British occupation, they disguise themselves as Arabs; and, accompanied by the wild Hazr brothers Ali and Mahmoud, they uncover a plot to destabilize the region.