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 Époque 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 Étretat. 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, Arsène Lupin, if he is to find the treasure and avert an international disaster at sea.