At a chaotic government employment center, Marianne first encounters Christèle, a financially beleaguered single mom with three children. She’s played by Hélène Lambert, who like most of the cast members is a nonprofessional actor. The presence of these untutored natural performers adds to the gritty believability of the film, which was fictionalized from Florence Aubenas’s nonfiction book, known in English as “The Night Cleaner.” (Two workers Aubenas chronicled actually play versions of themselves in the movie.)

The book was published in 2010, but its author for years resisted efforts to adapt it into a film. She finally relented, provided that it would be directed by Emmanuel Carrère, a novelist who’s made two movies, one of them a documentary. Carrère wrote the screenplay with Hélène Devynck, and they altered the story to highlight the role of the writer, who’s not central to Aubenas’s account.

As Marianne works a series of cleaning gigs, she’s befriended by several generous people, mostly women, including one who offers her a battered car. Marianne uses the vehicle to give Christèle rides to her way-out-of-town job at the local port, where she’s part of a team that must clean and refresh a France-to-Britain ferry in a mere 90 minutes. Workers who dawdle will find themselves trapped on the nine-hour voyage to Portsmouth — a predicament that will, of course, eventually occur.

Soon Marianne has joined the ferry custodial crew, and she becomes close to Christèle and her kids, as well as the younger Marilou (Léa Carne). As they clean toilets and make beds, the three women appear to be equals trapped near the bottom of the Gallic economy (although not so low as African refugees in limbo who are glimpsed near the port). But what might Christèle and Marilou think of Marianne if they knew why she was really there?

They inevitably find out, in a contrived scene. But it seems likely that the working-class women would have detected some caste or educational barrier between them and their new friend far sooner. Perhaps Marianne is meant to be as good an actor as Binoche. But the script’s crucial weakness is that the central characters are not drawn in sufficient detail. The screenwriters concentrate on the women’s shared experience of scrubbing and swabbing — humiliating and physically crushing — and provide only an occasional hint of the knowledge and experience that would separate a Parisian writer from provincial cleaners shaped by poverty and drudgery.

“Between Two Worlds” is freshest when it emphasizes its documentary-like qualities, such as the brief inserts of everyday scenes and locales shot by Philippe Lagnier without any guidance from the director. Less effective are traditional movie elements like Mathieu Lamboley’s score, which flirts too openly with Philip Glass’s style.

Binoche, who’s one of the film’s associate producers, is the dominant presence, even if she doesn’t appear in the poignant final shot. But it’s significant that the performer is alone in some of her most powerful moments, notably ones based on a loss the actress suffered while making the movie. These scenes are moving and indelible, but they also underscore the movie’s fundamental dilemma: It never really crosses the divide between the two worlds it attempts to depict.

Unrated. At Landmark’s E Street Cinema and the Cinema Arts Theatre. Contains strong language, drinking and smoking. In French with subtitles. 106 minutes.