At 221b Baker Street, Joanna Blalock and the Watsons receive an unexpected visitor, Dr. Alexander Verner, who has just returned from an unsettling trip to see a patient whom he believes is being held against his will. Joanna quickly realizes that Verner's patient is cryptographer Alistair Ainsworth, whom the Germans have taken captive to pry vital information about British military strategies for the Great War. In this third outing (after A Study in Treason), Joanna and the Watsons might be the only ones who can connect the clues to find Ainsworth before the Germans get the upper hand.