The film starts with the Kim family, a South Korean family struggling with poverty in a poor neighborhood in an unnamed city. Ki-taek and Chung-sook, the patriarch and matriarch, are having trouble finding employment, and their children, Ki-woo and Ki-jung are trying to help in whatever way they can. After struggling to make ends meet folding pizza boxes for a local pizza restaurant, the Kims are visited by one of Ki-woo's wealthier friends, Min-hyuk, who offers to refer him to a job tutoring for a wealthy family, the Parks.
Ki-woo visits the Parks in their gigantic home, where he is introduced to the spacey and out-of-touch Mrs. Park. He also meets Da-hye, the flirtatious teenager he will be tutoring, and Da-song, her younger brother, for whom Mrs. Park is trying to find an art tutor. Seeing an opportunity to get his artistically talented sister, Ki-jung, a job, Ki-woo says that he knows someone named "Jessica" who went to college in Illinois, who might be perfect for the job. Ki-jung gets the job after impressing Mrs. Park with some pseudo insights about psychology, and Mrs. Park hires her as both a tutor and an art therapist.
Soon enough, Ki-woo and Ki-taek secure positions in the Park household for their parents as well. They get Ki-taek a job as Mr. Park's driver, after Ki-jung leaves her underwear in the car to make it look like his previous driver was having a sexual relationship in the car. After hearing about the peach allergy of the housekeeper, Moon-gwang, Ki-jung and Ki-woo trigger an allergic reaction and make it look like she has tuberculosis. Chung-sook takes over as the housekeeper.
The Parks go on a camping trip for Da-song's birthday, leaving the house empty. The Kim family moves in for the weekend and luxuriates. It's all fun and games until Moon-gwang shows up, insisting that she left something at the house. The rest of the family hides as Chung-sook lets her in and Moon-gwang reveals a bunker in the basement of the house, a lair that was installed by the architect who built it, and which even the Parks have no knowledge of. Down in the basement is Geun-sae, Moon-gwang's husband, who is hiding from loan sharks there. Moon-gwang begs Chung-sook not to say anything to the Parks, but Chung-sook threatens to tell them.
Suddenly, the Kims fall down the stairs of the bunker in a pile, after listening in on the conversation. Realizing that they are all related, Moon-gwang takes an incriminating video of them and threatens to send it to the Parks. A fight breaks out in the living room of the house, and the Kims manage to get Moon-gwang and her husband down into the bunker, but suddenly, Mrs. Park calls and tells Chung-sook that they are heading home after the campgrounds flooded from a rainstorm. The Kims get trapped in the house, hiding under the coffee table when the Parks arrive home. When Moon-gwang tries to ascend the stairs and reveal herself, Chung-sook kicks her down, concussing and killing her.
When Da-song wants to sleep in a tent outside, Mr. and Mrs. Park sleep on the couch to watch over him. Ki-taek, who is hiding under the table, hears Mr. Park discuss his unsavory smell. Eventually, the Kims are able to escape, but when they return home, they find their apartment is completely flooded. The next day, Mrs. Park calls each of them in to work on Da-song's birthday party, which will take place on the lawn. When Ki-woo goes into the basement, Geun-sae attacks him and bludgeons him in the head with the rock. Geun-sae then emerges onto the lawn party, stabbing Ki-jung in the chest with a knife. As Da-song faints in fear, Mr. Park yells at Ki-taek to throw him the car keys, so that they can take Da-song to the hospital. When Ki-taek throws the keys, they land under Geun-sae, who is fighting with Chung-sook.
After Chung-sook stabs Geun-sae with a meat skewer, Mr. Park grabs the keys from under him, but in the process, scowls at Geun-sae's smell. This sets off Ki-taek, who stabs Park in the chest before running away and escaping into the basement bunker unseen.
Ki-woo wakes up in the hospital and learns about what happens. He stands in the woods near the Parks' house, which has been bought by Germans, and observes that the censor light is turning on and off in a pattern—Morse Code. He translates the code and realizes that it is a message from his father, who is living in the bunker. He writes his own letter back, in which he dreams about buying the house for himself and reuniting his family.
