Though it’s a phlegmatic, sometimes stumbling thriller, Moka, directed and co-written by Frédéric Mermoud, still has its share of gripping suspense. These tense moments arise not from any plot machinations but from the anticipation of the next exquisitely calibrated response by Emmanuelle Devos, the film’s star, who appears in every scene.

Devos plays Diane, a grief-sick woman determined to find the driver who killed her pubescent son in a hit-and-run six months earlier. A private detective has traced the automobile — Moka’s title comes from the vehicle’s coffee color — to Évian, the French town that’s a ferry ride away from the home in Lausanne, Switzerland, where Diane’s semi-estranged husband awaits her return. Diane has abandoned all bourgeois stability, bunking down in hotels or in her car’s back seat; her life is organized solely by her monomaniacal pursuit.

Moka’s first scene deftly communicates the depths of its heroine’s obsession. At some small hour, Diane approaches a window, surveying the majesty of Lake Geneva, the body of water shared by France and Switzerland that she’ll traverse a few times in the course of her sleuthing, and that Mermoud and cinematographer Irina Lubtchansky will return to in languorous pans. Slowly, steadily, she hits her head against the glass; this brief, wordless segment immediately signals Diane’s full awareness of her obsession and the potential futility of her quest.

Diane’s is a quiet, lucid derangement, vivified by Devos’s arsenal of infinitesimal gestures and physical responses; few performers share her skill at inhabiting long stretches of silence so absorbingly. The planes of the actress’s wide, square face and her enormous Bondi-blue eyes — pools as vast and deep as the lac so central to Mermoud’s movie — give Devos a saturnine mien, one that’s immensely helpful in establishing Diane’s despair. But beneath this superficial melancholy, more fervid instincts — revenge, bloodlust — churn.

As Diane seeks to fulfill those urges, though, Moka, based on a 2009 novel by Tatiana de Rosnay, becomes cluttered and somewhat incoherent, and its resolution owes more to syrupy maternal melodrama than to the Highsmithian mood aimed for (and sometimes achieved) in earlier scenes. But the desultory story picks up whenever Devos shares the screen with Nathalie Baye, as Marlène, the bottle-blond proprietor of a high-end parfumerie and Diane’s chief person of interest. These intergenerational performers — Baye began her career more than 40 years ago in films by Truffaut, Godard and Maurice Pialat — evince a fascinatingly protean chemistry, a slightly flirtatious vibe necessitated by Diane’s scheme to first cajole the woman she is convinced murdered her child.

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