A daring and delightful crossover of Sherlock Holmes and his criminal adversity: Arsene Lupin, the Gentleman Burglar. These superb sleuths will solve intricate riddles and journey across France and beyond to uncover the long-lost treasure of the House of Bourbon.

Sherlock Holmes and his cousin, Vernier, have been hired by the Baron of Creuse to find the legendary lost treasure of the kings of France.

Trekking from La Belle Epoque Paris to a chateau in the rural center of France, Holmes, Vernier and a new companion must employ all their wit to solve the fiendishly difficult puzzle of the Hollow Needle. After deciphering the meaning of the phrase "st. s. 138" and decoding a mysterious document, they realize the answer lies to the north in Normandy near the town of Etretat. Together, they follow a long-buried path to an ancient secret, but fresh mysteries and new complications immediately arise.

But other forces are at work, and jealous hands seek to interfere with Holmes's work. He must team up with the notorious gentleman-burglar, Arsene Lupin, if he is to find the treasure and avert an international disaster at sea.