The Kim family live 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 Kim's son, Ki-woo, pose as a university student to take over his job as an English language tutor for the daughter of the rich Park family, Da-hye. Ki-woo, posing as a Yonsei University student, is hired by the Park family.

The Kim family schemes to get each member a job with the Parks. The Kim daughter, 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 driver, by making it appear he had a sexual encounter in the car, then recommends her father, Ki-taek, to take his place. The Kims exploit the peach allergy of the Parks' long-time housekeeper, Moon-gwang, to convince Mrs Park that she has tuberculosis, and hires the Kim mother, Chung-sook, to replace her. 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 house. Moon-gwang 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. There, 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 Park family.

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 come home. 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 sexual intercourse on the sofa nearby. They overhear Mr Park's comments about Ki-taek's odour. The Kims escape and find their flat flooded with sewer water following the rainstorm, and shelter in a gymnasium with other displaced people.