One of the first things you hear in the sweetly melancholy new French film “Lover for a Day” is the sound of a young woman in the throes of sexual ecstasy. From her breathlessly enthusiastic shrieks, you wouldn’t necessarily guess that she and her partner were making love in a university restroom. The next sound you hear is of a different young woman weeping loudly in the street, with no one and nothing but a packed suitcase to keep her company.

There will be more lovemaking and more weeping to come in this latest picture directed by the veteran Philippe Garrel, one of France’s most celebrated post-New Wave filmmakers and a prolific, devoted observer of heterosexual relationships in all their foibles and frustrations.

In his moody, wispy romantic fables, set in a contemporary Paris shot in radiant black-and-white, beautiful young people fall in love, fall out of love and agonize over issues of sex, monogamy and fidelity. The talk is both earnest and sophisticated, the gender politics sometimes endearingly quaint. His work has a kind of willful naivete, an innocence that can prove enchanting and exasperating in equal measure.

“Lover for a Day,” which completes a thematic trilogy of sorts with Garrel’s “Jealousy” (2014) and “In the Shadow of Women” (2016), is one of his more enchanting specimens. The couple we meet in the first scene are a philosophy professor in his 50s named Gilles (Éric Caravaca) and his 23-year-old student, Ariane (Louise Chevillotte). Despite the judgment their relationship might attract from their peers, or from the more easily scandalized members of the audience, Garrel presents it without scorn or sordidness. He takes these lovers as seriously as they take each other.

Which is not to suggest that he in any way spares their feelings. The weeping woman turns out to be Gilles’ daughter, Jeanne (Esther Garrel, the director’s daughter), who has just had a bad breakup with her live-in boyfriend. Arriving on her father’s doorstep, she moves in and soon finds herself in cramped quarters with him and Ariane. The situation would be awkward even if the two girls were not the same age: Ariane gets upset when Gilles returns home one evening and bestows the first kiss on Jeanne. The lovers’ sex life cools too, when Gilles realizes Jeanne can probably hear them from the next room.

Gilles, ostensibly the fulcrum in this scenario, recedes into the background while the two young women take center stage, forging a real friendship in the process.

Jeanne, tempestuous and naive, is still heartbroken over the loss of her first love and contemplates suicide. Ariane stops her just in time. “You’ll get over it,” she says. “We always do.” But Ariane’s relative worldliness turns out to have its pitfalls too. She and Gilles have a more-or-less open relationship, allowing her to pursue the younger men she invariably meets around town.

Gilles prides himself on being secure enough to abide this arrangement, so long as he’s left unaware of the details, but like Jeanne and Ariane, he will eventually be confronted with the inadequacies of his plan, his worldview and his self-image.