A rich man walks up the stairs of his extravagant home. With each step, a lamp goes on above his head, lighting his path. He doesn’t think twice about why the lights are timed to his steps. He’s so used to always being in the light he hardly notices.

What’s going on with those lights is one small mystery among many in Bong Joon Ho’s brilliant social thriller “Parasite.” It’s the third film (after “Snowpiercer” and “Okja”) in which Bong turns the inequities of late capitalism into unnerving, darkly funny and constantly surprising high-end entertainment.

While “Snowpiercer” turned the class system into a series of train cars hurtling on an express ride to nowhere, “Parasite” turns the metaphor on its end. At the literal bottom of an unnamed South Korean city is the Kim family, crowded together in a semi-basement apartment that gives them a ringside seat to watch drunks peeing in the street.

The family scrapes by on menial jobs like folding pizza boxes, but teenage son Ki-woo (Choi Woo-sik) dreams of something better. A friend gives him a heavy stone that’s supposed to bring good luck, a metaphor for the elusive dream of upward mobility. If it’s so lucky, then why does it weigh him down so much?

But luck does come. A friend is off to college, and offers Ki-woo the chance to take his place tutoring the teenage daughter of a rich family who lives up the hill. As Ki-woo heads up to their mansion, the difference between his circumstances and theirs is literally night and day — the first time we see sunlight in the film is the moment when he enters their perfectly manicured backyard.

The house (actually a set built for the film) is an architectural marvel, a series of interlocking boxes with giant picture windows everywhere. But the windows only offer views of the grounds — giant hedges block the messiness of the outside world. The family pays a lot of money to forget that the rest of the world doesn’t live as they do.

Mr. Park (Lee Sun-kyun) and Mrs. Park (Cho Yeo-jeong) glide through life, pleasant but distant, so used to things going their way that they can’t imagine any other way of living. Ki-woo is entranced by their world, and soon convinces his sister Ki-jung (Park So-dam) to forge some credentials and get a job as an art therapist for the Parks' young son. Then comes mom and dad, hired on as the Parks' maid and personal driver.

This means finding some diabolically sneaky ways of getting the current staff fired, and as clever and funny as the schemes are, the laughs stick in our throat a little. We’re watching a family do whatever’s necessary to go from the bottom rung of the ladder to the second-from-the-bottom rung. Meanwhile, the rich family is oblivious — what’s one maid compared with another?

I won’t reveal any more, except that the family’s scheme to keep living the good life — or at least, to serve those who live the good life — is tested in ways they never imagined. Bong is brilliant at dispensing some wild plot twists, and shifting between several tones for the film. Some scenes have the daffy precision of a bedroom farce, others the claustrophobic tension of a home invasion thriller.

Bong handles these shifts confidently, and beneath the film’s visual wit and narrative elegance burns an undeniable anger at a world divided into Parks and Kims. Which family is the real parasite, feeding off the other, is a provocative question that lingers long after the movie’s ended and our heart rates have returned to normal.

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Rob Thomas is the features editor and social media editor for the Capital Times, as well as its film critic. He joined the Cap Times in 1999 and has written about movies, music, food and books.