Secret agent FX-18 goes undercover to break up an espionage ring.

Madame Andromèze's diamond bracelet has been stolen from the plane taking Nicole from Rome to Paris. Slipped by the thief into her bag, the bracelet undergoes many vicissitudes... Found and appraised, it is declared a fake! Nicole's brother and fiancé chase after the real bracelet, disrupting the life of a grand palace. Finally, Madame Andromèze finds her bracelet, which has been declared genuine. The original thief resurfaces and steals the jewel again.