During a critical stage in the Great War, South Africa's Governor-General is scheduled to present a precious blue diamond to King Edward as a symbolic gesture of the colonies' allegiance to England. Despite precautions, the diamond is stolen, and Scotland Yard is baffled. Making their sixth investigation (after The Abduction of Pretty Penny), Joanna and the Watsons are called in, and they deduce that the crime could greatly aid the Germans and turn the tide of war in their favor. Time is of the essence, and the Allies are counting on Joanna to come through.