Original title (FR): Reflet dans un diamant mort (Reflection in a dead diamond)

John D, a septuagenarian living in a luxury hotel on the Côte d’Azur, is intrigued by his roommate who reminds him of the craziest days of the Riviera during the 1960s. At that time, he was a spy in a growing world full of promise. One day, this neighbor mysteriously disappears… and brings John face to face with his demons again: are his adversaries of yesteryear back to sow chaos in his idyllic world?

https://www.themoviedb.org/movie/1013446-reflet-dans-un-diamant-mort