Franck (Pierre Deladonchamps), right, falls for Michel (Christophe Paou) despite obvious character flaws in “Stranger by the Lake.” (STRAND RELEASING)

Remember when “Basic Instinct” was such a shock to the senses, with its explicit sex and indifferent violence? How quaint. The French film “Stranger by the Lake” is an erotic thriller (heavy emphasis on the erotic) that makes that 1992 mystery look like “Nancy Drew.” What sets the engrossing “Stranger by the Lake” apart is that its excesses seem to point to a moral purpose beyond shock or entertainment value.

The lake of the film’s title is a vacation spot where gay men go to strip down, sprawl out, swim and, sometimes, venture into the woods for a bit of semi-private intimacy. Writer-director Alain Guiraudie takes an all-natural approach to his material, and not just because most of the men spend the movie in the buff. He takes long, lingering shots, never rushes a scene and uses no score, just organic sounds: the crunching of gravel underfoot in the parking lot, the gentle splashing of swimmers gliding across the lake, the gurgling as one man drowns another.

That last scene occurs at night as the protagonist, Franck (Pierre Deladonchamps), secretly looks on. A listless Adonis, Franck looks like a cross between an underwear model and a puppy with his sad eyes and earnest expressions. He’s between jobs, and he spends a lazy day trying to catch the eye of Michel (Christophe Paou), whose creepy nature and state trooper mustache nearly distract from his perfect, tan physique. (Another, funnier distraction: the big white tennis shoes Michel wears to stroll nude around the rocky beach.)

Later that night, when Franck witnesses Michel murder his lover, you’d think the young dilettante would either call the cops or run for his life. Instead, he starts a passionate relationship with Michel and protects the murderer when the body surfaces and a detective starts hanging around and asking questions.

Even with Guiraudie’s naturalistic depictions of revelry on the forest floor — the movie features many close-ups of various sex acts — there’s something surreal here, both in the way Franck falls for a murderer and the way Michel returns to the scene of his crime. The same goes for the relationship between Franck and another man, Henri. The pair talk as if they’ve known each other for ages when they’ve only just met.

In both of these relationships, Franck seems both intensely close to and extremely distant from his companions. No one talks about anything of substance, and each day consists of the same rigmarole of tanning, swimming and sex. The action is both strange and simplistic, qualities that make the film seem like it’s meant to be an allegory, but for what? It could be how some people insist on bad-idea relationships. Could there be a bigger red flag than watching your potential mate murder someone?

But the movie also calls to mind Hieronymus Bosch’s “The Garden of Earthly Delights.” Even when Franck and Michel say they love one another, they run off into the woods to be with others. Franck and all the other men loiter there, looking at each other as if they’re surveying confections in a bakery case. Franck’s interest in Michel seems to stem not solely from an attraction but also from a need for more excitement.

Nothing good can come of it, and not even a body in the lake can halt the men from congregating. This cold reality sometimes makes “Stranger by the Lake” feel more like an exercise than a story. But either way, it’s hard to forget.

★ ★ ★

Unrated. At Landmark’s E Street Cinema. Contains pervasive graphic nudity and sex, violence and language. In French with subtitles. 97 minutes.