The Kim family- son Ki-woo, daughter Ki-jung, father Ki-taek and mother Chung-sook- live in a semi-basement flat in Seoul, South Korea. Their struggles include losing WiFi, getting fumigated on, and witnessing drunk men piss outside their window. They make some money by folding pizza boxes, but their employer docks 10% of their pay due to Ki-taek's sloppy work. Ki-woo’s wealthy friend Min-hyuk, a university student, visits and gives the family a rock that is supposed to bring those who have it wealth and prosperity. Min-hyuk is leaving to study abroad and asks Ki-woo to take over his job as an English language tutor for Da-hye, the daughter of a very wealthy family called the Parks. Min-hyuk plans to propose to Da-hye once she graduates high school, and chose Ki-woo over any of his college friends because he doesn't want them to steal her from him. Ki-woo poses as a Yonsei University student with the help of Ki-jung's Photoshop skills to create a fake certificate for him. He gets hired by the Park family.
Mrs. Park mentions to Ki-Woo that they need an art tutor for their young son Da-song. Da-song had a traumatic incident where he saw a “ghost” in the house and had a seizure and has been needing help with his art. Ki-woo introduces them to “Jessica”, who is really Ki-jung, having forged documents for herself. She too gets hired and starts making money. Ki-jung purposely leaves her panties in the Park family's car, which Mr. Park finds and causes him to fire his chauffeur, believing he's been using the car for sex. Ki-jung suggests Ki-taek to be their new driver, claiming he's her uncle.
The Kims come up with an elaborate plot to get the Parks' housekeeper, Moon-gwang, fired. She has worked for the home since before the Park family lived there – she worked for the previous owner, an eccentric architect. The Kims exploit her allergy to peaches and frame it as tuberculosis. She leaves, devastated, and Chung-sook is given the job, fooling the Park family into hiring the entire Kim family. Ki-woo begins a secret romantic relationship with Da-hye.
When the Park family decides to leave to go camping for Da-song’s birthday weekend, the Kims take the opportunity to stay in the huge Park house for the weekend. Late at night, the doorbell rings, and it's revealed to be Moon-gwang, who claims she left something behind. She cut the power to the house's security camera so that no one would know she was there. Chung-sook lets her in, and she goes down into the basement, where she reveals a secret passage hidden behind some shelves to an underground bunker created by the previous architect homeowner (who was too embarrassed to inform the Park family of its existence). In the underground bunker is Moon-gwang's husband Geun-sae, who's been living in the bunker for four years to hide from loan sharks. He is the "ghost" that Da-song saw.
Chung-sook threatens to call the police, but Moon-gwang begs her to let Geun-sae stay, and even offers to pay her more money. Chung-sook declines and is about to call the police when the other three Kims, who were eavesdropping, accidentally slip down the stairs. Moon-gwang videotapes them on her phone inadvertently referring to each other by their family names. She threatens to send the video to the Parks, and uses that so she and her husband can force the Kim family to do their bidding.
They go back upstairs, but the Kims manage to get the upper hand and steal the phone. The Parks call and say that due to a severe rainstorm they're on their way back and will be home in 8 minutes. The Kims rush to clean up the house and keep Moon-gwang and Geun-sae in the bunker. Moon-gwang breaks out and runs back upstairs, but Chung-sook pushes her down the stairs, severely wounding her when she hits her head. Mr. and Mrs. Park end up sleeping in the living room in order to keep an eye on Da-song who is camping out in the backyard while Ki-jung, Ki-taek and Ki-woo hide under the living room table. Mr. Park comments about Ki-taek's smell, one he associates with lower-class people. They manage to sneak out unnoticed and return home to find their apartment completely flooded with rain and sewage. Ki-woo takes the rock, and the three of them take shelter in a gymnasium with other, similarly displaced people.
The next day, Mrs. Park decides to throw a party for Da-song with the elder Kims' assistance, while the younger Kims are invited as guests. Ki-woo goes down into the bunker with his rock to kill Geun-sae and Moon-gwang, but drops it down the stairs. He finds Moon-gwang has died from her head trauma and Geun-sae ambushes him, knocking him out with the rock. Geun-sae goes upstairs to the party and stabs Ki-jung in the chest in front of everyone. Seeing Geun-sae again causes Da-song to have another seizure. Chung-sook fatally impales Geun-sae with a barbecue skewer. Ki-taek tries to tend to Ki-jung, and sees Da-hye carrying an injured Ki-woo on her back. Ki-taek throws the car keys to Mr. Park, and they land right by Geun-sae. When Mr. Park once again expresses disgust at the way Geun-sae smells, Ki-taek kills him in a fit of rage, then goes to hide in the basement. He doesn't get spotted since Moon-gwang cut power to the security camera.
Weeks later, Ki-woo wakes up in a hospital. He and Chung-sook are convicted of fraud and put on probation. Ki-jung has died from her injury, while Ki-taek still has not been found. Ki-woo leaves the rock in a river and observes the former Park house, now owned by a German family, and sees a message in Morse code from a kitchen light. Ki-taek is now living in the bunker. He buried Moon-gwang in the garden, sneaks upstairs for food and sends the message every day, hoping Ki-woo will see it. Ki-woo writes his father a letter back, resolving that someday he will become rich enough that he can buy the house and their family can be reunited.
