After stumbling badly with the over-the-top “Okja,” Korean auteur Bong Joon-ho returns to form, and then some, with “Parasite,” a tour de force that straddles the line between comedy, melodrama, social satire, thriller and horror movie—a genre mash-up, to be sure, but one so artfully contrived and executed that although it runs over two hours, it’s hard to take your eyes off it for a minute.

The film begins with a portrait of the Kim family, who live a hand-to-mouth existence in a depressing half-basement apartment in Seoul, earning money by—among other things—folding boxes for a pizza-delivery service. There are four of them: sad-faced Ki-taek (Song kang-ho) and his wife Chung-sook (Chang Hyae-jin), a surly ex-shot-putter, and their children Ki-woo (Choi Woo-shik) and his sister Ki-jung (Park So-dam). The siblings are devastated when they lose the WiFi signal they’ve been cadging from a neighbor, and the family’s marginal finances are imperiled further when many of their pizza boxes are refused because of their shoddy appearance.

A hint of good fortune enters when Ki-woo’s friend Min-hyuk (Park Seo-joon) gives the family a landscape rock as a gift from his uncle, who swears it can bring them luck. He also offers Ki-woo a job replacing him as English tutor to Da-hye (Jung Ziso), the daughter of wealthy Mr. and Mrs. Park (Lee Sun-kyun and Cho Yeo-jeong); Min-hyuk is going to study abroad for a time, and needs someone he can trust to spell for him, since he’s interested in the girl and wants her left in relationship limbo while he’ gone. Securing the job requires Ki-jung, a talented forger, to confect university papers for Ki-woo, but the scam works.

Ki-woo quickly takes advantage of his position to secure various jobs for his family in the Park household. On his recommendation Ki-jung is taken on as the art instructor, named Jessica, for the Park’s hard-to-control, supposedly precocious son Da-song (Jung Hyun-joon). After she in turn gets Mr. Park’s chauffeur (Park Geun-rok) fired, Ki-taek is hired in his stead. He in turn persuades the Parks that their long-time housekeeper Lee Jung-eun (Gook Moon-gwang), who had previously served the famed architect who built their house, has an infectious disease. She’s promptly dismissed, only to be replaced by Chung-sook.

Thus far “Parasite” is a darkly comic take on the socio-economic divide in South Korea, with a family of genial grifters taking advantage of an upper-class clan. (One might note that Ki-woo betrays Min-hyuk as well, since despite his pledge to the contrary, he begins romancing Da-hye.) If that were all the film was about, one could chalk it up as a sharp, rather nasty satire and leave it at that.

But Bong takes us to a very different place when the Parks—who are depicted as absolute villains no more than the Kims are drawn as totally sympathetic (although the employers do take advantage of their hirelings and, on occasion, speak of them with obvious condescension)—go camping to celebrate Da-song’s birthday (a tradition to take the boy’s mind off the trauma he’d suffered when he encountered a ghost on that date a couple of years earlier). The Kims take over the house in their absence, treating it cavalierly as their private domain.

Their fun is suddenly shattered when a surprise visitor appears to threaten their fun—and more. “Parasite” now enters a thriller mode that, while laced with violence and menace (as well as shards of black farce), seems positively mild compared to the horrors that follow in the last act as economic and class divisions suddenly explode. A postscript introduces what might just be one curve too many, but that’s a minor quibble.

In the hands of a less capable director, “Parasite” would have stumbled from episode to episode, genre to genre, tone to tone, and wound up a mess. Bong, however, manages to shape it into a virtually seamless whole despite the radical changes the narrative makes along the way. It’s truly a virtuoso performance by a director whose control never fails or flags.

His cast appears to have been in complete harmony with his vision; the quality of the ensemble work is exemplary. Some occasionally play to the rafters, but that’s in line with the demands of the script (and Korean style). The technical side is equally strong, with sweeping camerawork by Hong Kyung-kwon and smooth editing by Yang Jinmo, and Lee Ha-jun’s production design, which creates a perfect dichotomy between the Kims’ dingy subterranean flat and the Parks’s austerely attractive mansion, is extraordinary: a sequence in which a flood strikes the poorer areas of the city is also astonishingly realized.

“Parasite” is a trenchant commentary on the economic divide that exists in today’s South Korea, one made all the keener by Bong’s ability to meld elements of comedy, satire, suspense and horror in presenting it. This is one of the year’s best films.