When the Keeper of the Jewel House at the Tower of London
discovers the rare 14th century King's Diamond missing from the Crown's collection, the Queen's Private Secretary, Colonel Alexander Crowhurst, calls Inspector Gregson in to recover the jewel. After a week with no leads, Queen Victoria privately commissions Sherlock Holmes to locate it, against the advice of her government. Holmes and Watson recognise that lost time has put them at a disadvantage, and now the great detective must bring his entire arsenal of skills to bear against a sinister foe whose own plans hinge upon preventing Holmes from returning the precious gem.