The Kim family lives in a semi-basement flat (banjiha) in Seoul, have low-income jobs, and struggle for money. Min-hyuk, a university student, gives the family a scholar's rock meant to promise wealth. Leaving to study abroad, he suggests that the Kims' son, Ki-woo, pose as a university student to take over his job as an English language tutor for Da-hye, the daughter of the rich Park family. After his sister Ki-jung helps create a false certificate for him in Photoshop, Ki-woo, posing as a Yonsei University student, is hired by the Parks.

The Kims scheme to get each of them a job with the Parks. Ki-woo recommends "Jessica", actually Ki-jung, as an art therapist for the Parks' young son, Da-song, who has been traumatised after seeing a "ghost" in their kitchen. Ki-jung frames Yoon, Mr Park's chauffeur, by making it appear he had a sexual encounter in the car, then recommends her relatives' chauffeur, in actuality her father, Ki-taek, to take his place. The Kims exploit the peach allergy of the Parks' longtime housekeeper, Moon-gwang, to convince Mrs Park that she has tuberculosis, and the Kim matriarch, Chung-sook, is hired as her replacement. Ki-woo begins a secret romantic relationship with Da-hye.

When the Parks take a camping trip, the Kims revel in the luxuries of the house. Moon-gwang appears at the door, telling Chung-sook she left something in the basement. She goes through a hidden entrance to an underground bunker created by the architect and previous homeowner (who was too embarrassed to inform the Parks of its existence). There, Moon-gwang's husband, Geun-sae, is revealed to be living in the bunker while hiding from loan sharks; he is also revealed to be the "ghost" Da-song saw attempting to come out of the bunker to steal groceries from the Parks' refrigerator. Chung-sook refuses Moon-gwang's pleas to allow Geun-sae to remain in the bunker in exchange for regular payments, but the three other eavesdropping Kims accidentally reveal themselves to be related. Moon-gwang videotapes them on her phone, inadvertently revealing their deception, and threatens to expose them to the Parks.

The Parks call to say that they are returning early due to a severe rainstorm. The Kims subdue Moon-gwang and Geun-sae, tying them up and hiding them in the bunker. Ki-jung, Ki-taek, and Ki-woo hide under a table, and overhear Mr Park's comments about Ki-taek's odor, one he associates with lower-class people. The Kims eventually escape, but the torrential rain floods their flat with sewer water, forcing them to take shelter in a gymnasium with other, similarly displaced people.

The next day, Mrs Park hosts a house party for Da-song's birthday with the elder Kims' assistance; the younger Kims are invited as guests. Ki-woo enters the bunker with his scholar's rock to kill Geun-sae and Moon-gwang. Moon-gwang is already dead from a concussion she received during the previous day's brawl, but Ki-woo is attacked by Geun-sae, who uses the rock to knock Ki-woo out, leaving him lying in a pool of blood in the basement. Geun-sae then stabs Ki-jung with a kitchen knife in front of the horrified guests. Da-song suffers another seizure upon seeing Geun-sae again. Geun-sae and Chung-sook grapple with each other; she fatally impales him with a barbecue skewer. While Ki-taek tends to Ki-jung, Mr Park orders him to drive Da-song to the hospital. In the chaos, Ki-taek, upon seeing Mr Park's disgusted reaction to Geun-sae's odor, angrily kills him with the knife, then flees.

Weeks later, Ki-woo is recovering from brain surgery. He and Chung-sook are convicted of fraud and put on probation. Ki-jung has died from her injury, and Ki-taek has disappeared. Geun-sae is assumed to have been a homeless man, and neither his nor Ki-taek's motive for the murders are known. Ki-woo spies on the Parks' former home, now owned by a German family, and sees a message in Morse code from a kitchen light. Ki-taek, hiding in the bunker, has buried Moon-gwang in the garden and sends the message every day, hoping Ki-woo will see it. Still living in their basement flat with his mother, Ki-woo writes a letter to Ki-taek, vowing to earn enough money to buy the house and free him.