The Aviator's Wife (1981)

A heartbroken student, François, becomes obsessed after suspecting his girlfriend Anne is cheating on him with her ex-lover, Christian, an airline pilot. When he sees Christian leaving Anne’s building, he begins spying on him. Spotting Christian with a blonde woman, François follows the pair, joined by Lucie, a curious 15-year-old who turns the pursuit into a playful investigation. Their theories spiral, but the dissapointing truth unravels later.