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According to a private detective, there are only four large, mocha-colored automobiles in all of Evian, a town on the French side of Lake Geneva. The protagonist of “Moka” — and the woman who paid for that information — lives across the water in Switzerland. She’s certain that one of those four cars struck and killed her child.

Adapted from a 2007 novel by Tatiana de Rosnay (whose grim “Sarah’s Key” was made into a movie in 2010), this is a stark, moody mystery that doesn’t actually contain much mystery. Instead, it excels as a character study and a dynamic faceoff between two formidable actresses: Emmanuelle Devos and Nathalie Baye.

Devos plays Diane, whose teenage son Luc (Paulin Jaccoud) died in a hit-and-run. Baye is Marlène, the woman who may or may not have been at the wheel.

Diane’s grief has left her estranged from her husband, Simon (Samuel Labarthe), who frustrates her by trusting in the police to investigate Luc’s death. Diane moves across the lake, accompanied only her son’s cellphone containing digital reminders of Luc and a girlfriend he never mentioned.

Eliminating the other suspects, Diane becomes convinced of Marlène’s guilt, Rather than confront her, Diane tries to insinuate herself into Marlène’s life. Marlène, who runs a beauty salon where Diane becomes a regular, is intrigued by the newcomer’s interest in her, but also suspicious.

As methodical as she is hysterical, Diane even attempts to buy the tan-colored car she believes struck Luc, which has been put up for sale by Marlène’s boyfriend, Michel (David Clavel).

Marlène is having trouble with her restless adolescent daughter Elodie (Diane Rouxel). Elodie is clearly out of control. In her quieter way, so is Diane. But neither one of them can be fixed by Marlène.

Swiss director Frédéric Mermoud presents this story as if it’s a real puzzler — which it isn’t, really. Attentive viewers will crack the case long before Diane does.

The Beethoven-heavy “Moka” ends with an affecting musical epiphany — both for Diane and one other person. It’s not the hit-and-run driver, but it’s someone Diane should have sought from the first.

Moka

★★½ out of 4 stars

Rating: Not rated but includes violence, nudity and sex. In subtitled French.

Theater: Lagoon.