Who died, and why? Those questions drive every murder mystery, and they are, indeed, at the heart of The Blue Room (originally titled La chambre bleue), a superb new film from actor/director Mathieu Amalric, based on a novel by Georges Simenon.

Before those questions are raised, Amalric sets the tone for a languid, sensual affair between Julien Gahyde, the character he plays, and Esther Despierre (Stéphanie Cléau), an old friend. Julien is comfortably married to Delphine (Léa Drucker), with whom he has a bright young daughter, Suzanne (Mona Jaffart). They enjoy a settled lifestyle in a beautiful home near the water. In the nearby small town, Esther is a pharmacist; she lives upstairs, discontentedly, with her husband. Julien and Esther begin meeting once a week for sexual pleasures, quietly expressing an apparently mutual desire to be together forever.

Things don’t work out quite that way, as made apparent early in the film. Julien is in custody and undergoing extensive questioning by the police. Who died, and why? As noted, those questions are at the heart of the story, but Julien’s mind is occupied by the past, by memories of Esther and warm afternoons spent naked in bed with her, by recollections of Delphine and the time they spent together and the off-handed way he treated her, by reminiscences about Suzanne, the true love of his life.

The unanswered questions burble in the background, occasionally fighting their way to center stage, but mostly allowing the more fascinating issue of character to come to the fore. Julien appeared to have everything: a beautiful wife, a beautiful child, a beautiful home, and a respectable job earning more than enough money. What drove him into the affair? He and Esther knew each other when they were much younger, yet they never consummated their interest. Was that yearning somehow burning in their loins for lo so many years? Were they both biding their time, hoping that their paths would cross again sometime in the future?

Amalric is best known in the U.S. as a very busy actor, but he’s made time to direct his own projects in France for years, and that experience results in a finely-tuned movie. It’s spare and distant, and maybe even a little chilly at times in its emotional restraint, but that works to the story’s abundant advantage when there is an outburst; it feels shocking and true. Visually, every shot feels well-chosen and fresh, without the distraction of bombastic music signaling the ‘proper’ response to the scene.

As an actor, Amalric effectively varies his own visage; his character is generally restrained in expressing emotion toward his wife, yet is much more open with his mistress, as evidenced by the markedly different physical nature of the relationships. Only with his daughter, however, can Julien be seen without affectation; it feels like the only pure relationship he’s ever had.

The rest of the cast toes a similar line; it feels like everyone is exercising self-control in support of the inner turmoil that so many are experiencing. That makes The Blue Room a rare and unexpected treat: a movie that builds in complexity one scene at a time, while storing up emotional power that never quite dissipates.

The film opens at the Angelika Film Center in Dallas on Friday, October 24.