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.
Diane's is a quiet, lucid derangement, vivified by Devos' 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 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 Tatiana de Rosnay's 2009 novel, becomes cluttered and somewhat incoherent. Its resolution owes more to syrupy maternal melodrama than to the Highsmithian mood aimed for (and sometimes achieved) in earlier scenes.