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), “Moka” is a stark, moody mystery that doesn’t actually contain much mystery.

Instead, it excels as a character study and a dynamic face-off between two formidable actresses: Emmanuelle Devos and Nathalie Baye.

Devos plays Diane, whose teenage son Luc (Paulin Jaccoud) has 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, for her part, moves across the lake, accompanied only by a cellphone containing digital reminders of Luc and the girlfriend he never mentioned, Adrienne (Marion Reymond). On the ferry over, Diane meets a young smuggler (Olivier Chantreau). His specialty seems to be dope, but he just might also be able to get Diane a gun.

Quickly eliminating the other suspects, Diane becomes convinced of Marlène’s guilt. But rather than confront her, Diane begins by trying to insinuate herself into Marlène’s life.

As methodical as she is hysterical, Diane even attempts to buy the tan-colored Mercedes she believes struck Luc, which has been put up for sale by the 60-something Marlène’s younger boyfriend, Michel (David Clavel).

Marlène, who runs a beauty salon where Diane soon becomes a regular, using an assumed name, is intrigued by the newcomer’s interest in her, but also suspicious.

The two might have bonded over maternal pressures, since Marlène is having trouble with her restless adolescent daughter Elodie (Diane Rouxel). But Diane tells Marlène she’s not a mother.

Elodie is clearly out of control. In her quieter way, so is Diane. (“Stop being so reasonable,” she implores Simon.) Neither one of them can be fixed by Marlène or Michel, although both are menders of female psyches: Marlène with creams and lotions, Michel with the aquatic exercise classes he leads at a local spa.

Swiss director Frédéric Mermoud is no Hitchcock, yet he presents this story as if it’s a real puzzler, even though there are no MacGuffins to throw off amateur sleuths.

Attentive viewers will crack the case long before Diane does.

Discovering who did what to whom isn’t the point anyway. Luc and Adrienne studied music together, and 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.