Arsène Lupin is opposed this time by Isidore Beautrelet, a young but gifted amateur detective, who is still in high school but who is poised to give Arsène Lupin a big headache. In the Arsène Lupin universe, the Hollow Needle is the second secret of Marie Antoinette and Alessandro Cagliostro, the hidden fortune of the kings of France, as revealed to Arsène Lupin by Josephine Balsamo in the novel The Countess of Cagliostro (1924). The Mystery of the Hollow Needle hides a secret that the Kings of France have been handing down since the time of Julius Caesar... and now Arsène Lupin has mastered it. The legendary needle contains the most fabulous treasure ever imagined, a collection of queens' dowries, pearls, rubies, sapphires and diamonds... the fortune of the kings of France.
When Isidore Beautrelet discovers the Château de l'Aiguille in the department of Creuse, he thinks that he has found the solution to the riddle ("l'Aiguille Creuse" being French for "The Hollow Needle", and also the French title of the novel). However, he did not realize that the château was built by Louis XIV, the king of France, to put people off the track of a needle in Normandy, near the town of Le Havre, where Arsène Lupin, known also under the name of Louis Valméras, has hidden himself.

Question: What is young amateur detective Isidore Beautrelet in search of?
Answer: The secret of the Hollow Needle. 
Alternative answer: The solution to the Hollow Needle.