The Kim family—father Ki-taek, mother Chung-sook, daughter Ki-jung, and son Ki-woo—live in a small semi-basement apartment (banjiha) in Seoul, have low-paying temporary jobs folding pizza boxes, 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 and knowing his friend needs the income, he suggests that Ki-woo pose as a university student to take over his job as an English tutor for the daughter of the wealthy Park family, Da-hye. Ki-woo, pretending to be a Yonsei University student, is subsequently hired by the Parks.

The Kim family schemes to get each member of the family a job by posing as unrelated and highly qualified workers to become servants of the Parks. Ki-jung poses as "Jessica" and, using Ki-Woo as a reference, becomes an art therapist to the Parks' young son, Da-song. Ki-jung frames Yoon, Mr Park's chauffeur, by making it appear as if he had sex in the car, then recommends Ki-taek 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. Ki-woo begins a secret romantic relationship with Da-hye.

When the Parks leave on a camping trip, the Kims revel in the luxuries of the residence before Moon-gwang abruptly appears at the door, telling Chung-sook she left something in the basement. She enters a hidden entrance to an underground bunker created by the architect and previous homeowner, where Moon-gwang's husband, Geun-sae, has been secretly living 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 films them on her phone and threatens to expose their ruse to the Parks.

A severe rainstorm brings the Parks home early, and the Kims scramble to clean up the home and subdue Moon-gwang and Geun-sae before they return. The Kims trap Geun-sae and 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" — actually Geun-sae — emerging from the basement at night. Ki-jung, Ki-taek and Ki-woo hide under a table within earshot of Mr and Mrs Park who have sex on the sofa nearby. Before the three manage to sneak out of the house, they also hear Mr Park's off-handed comments about Ki-taek's smell. The Kims find their apartment flooded with sewer water as a result of the severe rainstorm 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 find Geun-sae. Finding Moon-gwang has died from a concussion she received during the earlier fight, he is attacked by a deranged Geun-sae, who bludgeons his head with the rock and escapes, leaving Ki-woo lying in a pool of blood at the entrance to the basement. Seeking to avenge Moon-gwang, Geun-sae stabs Ki-jung with a kitchen knife in front of the horrified party 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.