With her third film “Sibyl,” French director Justine Triet explores the transgression of boundaries. These barriers exist between the home and the workplace, they separate fiction and reality and regulate the flow of memories from the past to the present. This time around, Triet uses the thriller genre as a vehicle for this exploration of a woman’s experience. One of four female directors selected for the Main Competition of this year’s Cannes Film Festival, Triet, unfortunately, delivers a flimsy, unremarkable story of obsession unlikely to reach much further than local cinemas.

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The Sibyl of the film’s title, portrayed by Virginie Efira, is a psychologist weaning off her patients to return to her first love of writing fiction. She can’t resist the lure of hysterical patient Margot (Adèle Exarchopoulos, perpetually weeping), an up-and-coming leading lady having an affair with the principal actor of her next project (Gaspard Ulliel), also the boyfriend of its director (Sandra Hüller). Unwittingly swept up into Margot’s drama, Sibyl’s own demons return to haunt her and challenge her sobriety and the stability of her household.

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Already explored in her previous movies “The Age of Panic” and “In Bed with Victoria,” Triet has a vested interest in the experience of mothers who have to balance their professional lives and motherhood. This engaging, deeply-felt theme is regrettably smothered by the fractured focus of “Sibyl,” ironically taking on a clarity when the novelist finds herself at her lowest point.

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Recollections present themselves like the stray thoughts of a fraying mind. The relationship between the current and past tense is difficult to pin down over the film’s first half, as the narrative comes to clarify the triggers of Sibyl’s nostalgia. At least the main character’s crisis is more interesting than that of Margot, which the shrink-cum-author unethically uses as material for her comeback novel. Both Exarchopoulos and a smug Ulliel come across as unlikable in their performances, their romantic entanglement banal and its deployment trite.

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In her tragicomic turn as the titular character, Efira is very much on the same wavelength as Triet’s vision in their second productive collaboration. Sibyl is composed until she crumbles, presenting a brave face to those that depend on her. Only in the flashbacks with former flame Gabriel (Niels Schneider) does she drop her mask and surrender herself completely, and here wild behavior in the throes of young love set a precedent for future behavior.

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Among a starry cast curated to best pop out on a French one-sheet, German actress Hüller is the real scene-stealer. It’s a canny and appreciated choice to make the director of the fictional “Never Talk to Strangers” female; in all other details, the film within the film isn’t all that convincing, despite an out-of-focus Berlinale icon on its poster. As with her delivery in “Toni Erdmann,” Hüller is hilarious as a relative outsider to the banal central drama and left with the responsibly of keeping a production going under great tension.

As a thriller about obsession, “Sibyl” isn’t nearly lurid enough to satisfy. It’s a nonsuccess at reveling in the genre’s delicious style; cinematographer Simon Beaufils, who sunk his teeth into Brian De Palma-esque trashiness with last year’s “Knife + Heart,” shoots this film in the bland house style of popular French cinema. The location shooting in Stromboli—a surefire way to pay homage to the eponymous Roberto Rossellini film with its rich psychology, or Jean-Luc Godard’s lush “Contempt”—is a waste.

The only space in the movie that registers is Sibyl’s claustrophobic apartment; its scarlet red walls shrink a space-starved for natural light and densely populated with books and other bric-a-brac. This domestic space is one of many insights into the protagonist’s inner-workings: her anxious vaping, an uneasy bond with her clingy sister and deceased mother, the routine of the AA meetings. But these hints don’t resonate, betrayed by the film’s meandering, tonally unfocused construction. Margot’s only other remaining patient, a young boy processing the trauma of his mother’s death, remarks: “I don’t understand your mind.” Triet’s film might lay clues to navigate Sibyl’s thoughts, but ultimately, we don’t understand either. [C]

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