Two state employees, Molot and Maurent, are very fond of detective stories. They open a private eyes agency, which only opens on Sundays. But the clients are rare, and some kind of Arsène Lupin steals their savings. They set out to track so called suspects down but everything backfires on them; however,one fine day, a man knocks on their door: he asks them to shadow his fiancee who may (or may not) be true to him. Maurent falls for her.