After being asked to resign from the FBI, a publicity-hungry detective goes into private business. His first job is to protect a very precious jewel belonging to the Grissac family, which is the object of a failed robbery attempt in New York City. When he accompanies the Grissacs back to France, he encounters a friend of the family, Rene Farrand, who he rapidly comes to suspect is the master thief Arsène Lupin, someone believed to have been killed several years before.
