Arsène Lupin has assumed the identity of Baron d'Enneris. Accompanied by Cissy Chevalier, who has concealed her profession as a journalist, he arrives in Locarno. There, he finds himself in the midst of a family drama: Adelina Da Ponte's wedding cannot take place because the famous papal medallion, a fabulous jewel and ancestral talisman, has been stolen. Everything seems to point to Adelina's brother Aurélio, who has disappeared. The medallion reappears around the neck of the beautiful Valérie Daubenton, wife of an international financier.