A chance find of a gold pocket watch at the Reichenbach Falls brings Holmes and Watson to the 1902 'Paris in London' exhibition. The watch belonged to none other than the evil James Moriarty and is seen, by Holmes, as a way to exploit his perception of the Achilles heel of his equally wicked sister, Julia Moriarty. Indeed, Holmes believes the watch to be "The Holy Grail", the one thing that Julia Moriarty MUST possess.

Holmes must now devise a grand deception that is totally without guile, using the watch to lure Julia Moriarty to her destiny with the gallows. To accomplish this, Holmes requires assistance from an agent of the French Deuxieme Bureau and his plan unfolds in a series of masterful deceptions. The majestic Ferris wheel at the 'Paris in London' Exhibition now becomes the backdrop for the climactic finale, but who will be the victor? Will the ever resourceful Julia Moriarty triumph over Holmes?