The Kim family—father Ki-taek, mother Chung-sook, daughter Ki-jung and son Ki-woo—live in a small semi-basement apartment (banjiha), have low-paying temporary jobs as pizza box folders, and struggle to make ends meet. University student Min-hyuk, a friend of Ki-woo's, gives the family a scholar's rock meant to promise wealth. Leaving to study abroad, he suggests that Ki-woo take over his job as an English tutor for the daughter of the wealthy Park family, Da-hye. Ki-woo poses as a university student and is hired by the Parks, who call him "Kevin".
The Kim family recommends one another as unrelated and highly qualified workers to take over as servants of the Parks. Ki-woo tutors and seduces Da-hye. Ki-jung poses as "Jessica", an art therapist to the Parks' young son, Da-song. Ki-jung frames Mr. Park's chauffeur for having sex in the car by leaving her underwear there, and Ki-taek is hired to replace him. Finally, Chung-sook takes over as the Parks' housekeeper after the Kims exploit the peach allergy of the long-time housekeeper, Moon-gwang, to convince Mrs. Park that she has tuberculosis.
When the Parks leave on a camping trip, the Kims revel in the luxuries of the Park residence. Moon-gwang appears at the door, telling Chung-sook she has left something in the house's basement. Through a hidden entrance to an underground bunker created by the house's architect and previous owner, it is revealed that Moon-gwang's husband, Geun-sae, has been secretly living underneath the home for over four years, hiding from loan sharks. Chung-sook refuses Moon-gwang's pleas to help Geun-sae remain in the bunker, but the eavesdropping Kims accidentally reveal themselves. Moon-gwang threatens to report them to the Parks.
A severe rainstorm brings the Parks home early, and the Kims scramble to clean up the home before the Parks return, while a brawl breaks out between Moon-gwang, Geun-sae, and the Kims. The Kims trap Geun-sae and a mortally wounded Moon-gwang in the bunker. Mrs. Park reveals to Chung-sook that Da-song had a seizure-inducing traumatic experience on a previous birthday, when he saw a "ghost" (Geun-sae) emerging from the basement. The Kims manage to sneak out of the Parks' house, but not before hearing Mr. Park's off-handed comments about how Ki-taek smells bad even though he praises Ki-Taek's driving skill and experience. They find their home flooded with sewer water and are forced to shelter in a gymnasium with other displaced people.
The next day, Mrs. Park hosts a house party for Da-song's birthday with the Kim family's help. Ki-woo enters the bunker with the scholar's rock to face Geun-sae. Finding Moon-gwang dead, he is attacked by Geun-sae, who bludgeons him with the rock and escapes. Seeking to avenge Moon-gwang, Geun-sae stabs Ki-jung with a kitchen knife in front of the horrified guests. Da-song suffers another seizure upon seeing Geun-sae, and a struggle breaks out until Chung-sook fatally impales Geun-sae with a barbecue skewer. While Ki-taek tends to a severely bleeding 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 smell, takes the knife and kills Mr. Park before fleeing the scene.
Weeks later, Ki-woo wakes up after brain surgery. He and Chung-sook are convicted of fraud and put on probation. Ki-jung has died from her injury and Ki-taek, wanted for Mr. Park's murder, has vanished. Geun-sae's motive for the attack is not found. Ki-woo watches the Parks' home, sold to a German family unaware of its history, and sees a message in Morse code from a flickering light. It is from Ki-taek, who escaped into the bunker via the garage and now raids the refrigerator at night. It is also revealed that he had buried Moon-gwang in the backyard. He has been flicking the light every day, hoping Ki-woo will see it. Still living in the banjiha with his mother, Ki-woo writes a letter to Ki-taek, vowing to earn enough money to one day purchase the house and free his father.