NOTE: This spoiler was submitted by Alex
Ki-woo Kim (Choi Woo-Shik) is a young man living in poverty in the slums with his family – father Ki-taek Kim (Song Kang-ho), mother Chung-sook Kim (Jang Hye-jin), and sister Ki-jung (Park So-dam). The family lives in a garden unit where they struggle finding a WiFi signal to sneak into, get fumigated on, and have to watch men pissing in the alley outside their home. They make ends meet by doing menial tasks such as folding pizza boxes – and even then, get criticized by the pizza employees for messing up the boxes. Ki-woo’s wealthy friend Min-hyuk pays the family a visit, giving them a gift – a rock that is supposed to bring those who have it wealth and prosperity. He then tells Ki-woo that he has been tutoring the teenage daughter of a very wealthy family, He is leaving to study abroad but is in love with the daughter, and knows that any of the other university boys would steal her away. He wants Ki-woo to be her tutor, knowing he will watch over her so that Min-hyuk can propose to her once she graduates high school. Ki-woo knows he isn’t qualified since he isn’t in college, but Min-hyuk promises to vouch for him, and so he agrees and has Ki-jung forge accreditations for him to take to his interview.
Ki-woo interviews at the very wealthy Park family – Mr. Park (Lee Sun-kyun), scatterbrained Mrs. Park (Choi Yeo-jeong), and their children, teenage daughter Da-hye and young son Da-song. Ki-woo realizes he needs to flirt with Da-hye to get the job, and he does – Mrs. Park pays him an exorbitant amount of money and mentions they need an art tutor for 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 Ki-jung, who forges documents for herself and goes by “Jessica”, and she too begins making money hand over fist. When the Park’s driver takes her home, Ki-jung leaves her underwear in the car in order to get him fired: she then suggests her “Uncle” as the new driver – who is really Ki-taek. The last position is that of the housekeeper, Moon-kwang (Lee Jeung-eun). She has worked for the home since before the Park family lived there – she worked for the previous owner, an eccentric architect. In order to get her fired, the Kims exploit her allergy to peaches, causing her to have allergic reactions: they then convince Mrs. Park that she is seriously contagiously ill and cannot be around their son. She leaves, devastated, and Mrs. Kim is given the job, fooling the Park family into hiring the entire Kim family.
The Kim family enjoys their huge increase in income, and when the Park family decides to leave to go camping for Da-song’s birthday weekend, they take the opportunity to stay in the huge Park house for the weekend. They spend the evening drinking and eating and making a mess of the place, when the doorbell rings: it’s Moon-kwang. She claims she was fired so quickly she left without being able to get something and just wants it back. Mrs. Kim reluctantly lets her in, and Moon-kwang runs into the basement and begins screaming, opening a secret passage behind some shelves. She goes to her husband, Geun-sae (Park Myung-hoon), who has secretly been living in the bunker every since the previous owner moved out in order to hide from loan sharks. She gives him food while Mrs. Kim looks on in horror – she tells Moon-kwang she needs to leave, and as Moon-kwang begs her to let them stay, the rest of the Kim family (who had been eavesdropping) falls off the stairs and into view – and they call each other “dad”, etc, which Moon-kwang films on her phone, realizing the con the family has pulled. She threatens to send it to the Kims, and uses that so she and her husband can force the Kim family to do their bidding.
The Kim family manages to get the upper hand on them, getting them into the secret bunker, but the Park family calls: they’ve canceled their camping trip due to rain and will be home in eight minutes. The Kims scramble, trying to clean up as much mess as they can, keeping the other two in the basement. They manage to do a good enough job that the rest of the family is able to hide while Mrs. Kim gives the Parks their dinner – when Moon-kwang breaks out and runs upstairs, Mrs. Kim shoves her back down the stairs, where she hits her head and is severely wounded. Moon-kwang and Geun-sae are locked in the bunker. 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, forcing the Kims to stay under the table, frozen all night, even as the Parks complain about Mr. Kim’s smell – and then later, have sex. Eventually, in the dead of night, they are able to sneak out. They return home to find their apartment completely flooded with rain and sewage. Ki-woo takes the rock, and the family sleeps in a shelter for the night.
The next day, Mrs. Park decides to throw an impromptu party for Da-song. The Kim family, in their roles as help, are invited and have to pretend that they don’t know that there are two people locked in a bunker under the house. Ki-woo takes the rock down into the bunker, where Moon-kwang has died, and is ambushed by Geun-sae, who bludgeons him in the head with the rock. He then enters the party, where he stabs Ki-jung in the chest. The party explodes into horror, and Da-song has a seizure – Geun-sae was the “ghost” he had seen in the house prior. Mr. Park screams at his driver, Mr. Kim – who is trying to stop Ki-jung’s bleeding – for the car keys to take his son to the hospital, and he throws them to him. They land under Geun-sae, who is fighting with Mrs. Kim. She manages to kill him by stabbing him with a meat skewer. Mr. Park gets the keys, but expresses disgusts at Geun-sae’s smell – this triggers Mr. Kim, who snaps and stabs Mr. Park, killing him. Mrs. Park faints as Mr. Kim flees.
Ki-woo wakes up in the hospital, where he had been in a coma for weeks. He finds out that Ki-jung has died, and he and Mrs. Kim are sentenced to probation. There has been no sign of Mr. Kim, even though the police have been searching for him for Mr. Park’s murder far and wide. Ki-woo leaves the rock in a river and observes the former Park house where he sees the lights flickering – Ki-woo translates the flickering from morse code, and learns Mr. Kim is controlling them from inside the bunker, where he is now living, sneaking upstairs for food from the new owners. 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.