At first glance, the protagonist of the enjoyable sex-and-death-and-Instagram dark comedy “Rotting in the Sun” would appear to be its director: the 44-year-old Chilean-born filmmaker Sebastián Silva (“Nasty Baby,” “Crystal Fairy”), here playing a severely depressed version of himself. He enters the movie in an existential funk, his head full of suicidal thoughts and his sad-eyed, strikingly handsome face buried in a copy of E.M. Cioran’s “The Trouble With Being Born.” Soon he flees his Mexico City apartment for a gay nude-beach paradise, but Sebastián already seems dead to life’s pleasures, whether he’s taking in a sunset or snorting ketamine. Even the many, many naked men parading into his line of sight (and ours, thanks to Gabriel Díaz Alliende’s unabashedly horny, often crotch-level camerawork) seem to fill him with more gloom than delight.

If you crave happier company, you’re in luck: Enter the comedian and social-media personality Jordan Firstman, also playing a fictionalized version of himself, though not a severely depressed one. An exuberantly narcissistic human firecracker, Jordan makes a dynamic first impression on Sebastián — let’s just say he nearly kills them both — and thereafter remains a irrepressible, sometimes unwitting agent of clothing-optional chaos. Fate brought them together, Jordan claims, and he insists that they work together on the show he’s writing. The details of the project are vague, but it will involve turning a spotlight on some of his kajillion Instagram followers, the ones who hang on every pointless life update and semi-inspired comic bit he uploads.

The show sounds dreadful, but its working title (“You Are Me”) and pass-the-mic premise do raise a fascinating question: Is Jordan in fact the real protagonist of this movie? The argument could be made, especially when Jordan, keen to collaborate with Sebastián in every sense, follows him home to Mexico City, only to stumble on a mystery of ludicrous yet weirdly plausible proportions. I won’t say more (“Rotting” doesn’t need spoiling), except to note that Jordan swiftly seizes control of the narrative and turns amateur detective, interrogating Sebastián’s landlord, Mateo (Mateo Riestra), and housekeeper, Vero (Catalina Saavedra), and siccing his fellow Instagram sleuths on a puzzle to which the audience has already been given the solution.

And so “Rotting in the Sun” unfolds like an influencer-skewering episode of “Columbo,” if “Columbo” had smartphones, jaggedly kinetic visuals and unsimulated gay sex scenes. The last have predictably dominated headlines about the movie since it premiered at this year’s Sundance Film Festival, though Silva, who co-wrote the script with Pedro Peirano, could hardly be accused of trading in shock value. Even when he strategically frontloads the full-frontal at that beach getaway, his aim is less to eroticize than to normalize the sight of proudly, casually bared flesh. He treats sex as both life-animating force and banal matter-of-fact reality, a wellspring of nonstop hilarity and soul-crushing disappointment alike.

Sebastián’s suicidal ideations — he can’t stop Googling “phenobarbital” — suggest a deep familiarity with that disappointment. Jordan, by contrast, is an eternal optimist; there’s no frustration that a random hook-up and a few thousand likes can’t chase away. What holds the movie together, for all its jittery syntax and rug-pulling midpoint twist, is the furiously combative, contrapuntal energy that courses between Silva and Firstman. Both actors gamely mock themselves and each other, upending our ready-made assumptions about the value of the obscure, self-serious artist relative to the vacuous, hugely popular influencer. (Naturally, it’s only Sebastián’s reluctant mention of a potential Jordan Firstman collaboration that stirs any excitement at a depressing HBO pitch meeting.)

As ever, Silva’s filmmaking — formally rough on the surface, carefully worked out underneath — depends on the steady upending of expectations. Social media is phony but potentially revealing. Bodies are hot and sexy until they’re gross and inconvenient. Jordan is insufferable, the worst kind of self-entitled Ugly American, but also endearing, perceptive and admirable in his tenacity. What happens to him and Sebastián is harrowing, then funny, then shocking, then sad, then somehow all the funnier for it. A master at orchestrating domestic chaos in close quarters, Silva turns Sebastián’s under-construction apartment building into a den of vice, a warren of secrets and a locus of those unspoken class tensions and disparities that have always festered away in the director’s work.

At times, “Rotting in the Sun” plays like a Silva greatest-hits compilation, laced with references to everything from the great-outdoors trippiness of “Crystal Fairy” (explicitly cited by Jordan himself) to the self-skewering satire of “Nasty Baby.” The most significant callback, however, is to Silva’s 2009 drama, “The Maid,” starring Saavedra as a housekeeper waging quiet and not-so-quiet war against her longtime employers. She brings a different but similarly subversive spirit to the role of Vero, who initially registers as a kind but hapless peripheral figure, but who increasingly becomes a thorn in Jordan’s side as she refuses to let him seize the upper hand.

Vero, infuriating and sympathetic by turns, has little money and no social-media profile to speak of. In a movie defined by its bizarre meta-flourishes and bacchanalian pleasures, she’s hardly the first person you notice. But by the end of “Rotting in the Sun,” you may well conclude that this desperately sad, shifty-eyed woman — unnoticed, unloved and wholly unforgettable — has been the true protagonist all along.