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When someone commits murder, who’s crazier: the killer or the person who loves him? Alain Guiraudie’s Stranger by the Lake asks this question in a frequently mesmerizing and extremely adult mystery set on an isolated French beach known for cruising. If you prefer to know a character’s motivation or have any hint of his background or personality, though, you won’t find it here. This is a story as blank-slate as the anonymous sex that occurs in the nearby woods. It’s quiet and largely passive, a nearly thrillless thriller.

Stranger by the Lake is also a sticky floor away from being porn with a plot. On the beach, men lounge naked, with legs spread and Guiraudie’s camera aimed where the sun don’t shine. Among the trees, the action—also not of the thriller variety—is explicit. Be prepared for extreme close-ups of blow jobs, masturbation, penetration, fingering, and all the associated fluids. Friday the 13th this is not.

The film centers on Franck (Pierre Deladonchamps), a young man with boyish good looks about whom we know nothing, except that he’s between jobs. (Even names aren’t revealed until halfway through, which is irritating but appropriate to the milieu.) One day as the sun is setting, he emerges from his latest encounter to see two men in the lake, one dunking the other seemingly playfully until the dunkee vanishes. The murderer is Michel (Christophe Paou), a Magnum, P.I. doppelganger who then dresses and stoically heads home.

Guiraudie films this four-minute scene in one take, as if the camera were Franck’s eyes. It’s shot at quite a distance, as is much of the film: Voyeuristically, we watch men watching the man whose perspective is shown, as he takes in what’s going on around him. The entire film is set outdoors, with nothing but an ambient soundtrack that gets impressively ominous, particularly trees rustling in the wind nearly as loud as a storm. Even the lack of musical and its emotional cues, combined with not a whole lot of dialogue, is unsettling.

Obviously, this is not a whodunit. The tension—and a head-scratching question mark—occurs when Franck couples with Michel, apparently thinking his cheesy ’70s look is sexy. Is Franck’s infatuation so great that he doesn’t care about Michel’s crime? Is danger part of the allure? It’s nearly impossible to tell; even when Franck talks to others about the relationship, he gives no clues. The absence of his rationale is infuriating.

Accept the unexplained, however, and you may get caught up in the film’s increasing suspense. It’s impossible for quiet to get quieter, yet it seems to, as characters lurk in the woods trying to spy on—or hide from—each other. Many scenes are shot in the dark, including the final sequence, with Guiraudie opting for an open rather than Hollywood end. You’ll squint trying to see what’s going on, perhaps holding your breath. Then the filmmaker tacks on one final odd development, ensuring your continued engagement with a story that started out seeming so simple.