Kim Ki-taek and Chung-sook live in a cramped semi-basement flat (banjiha) in Seoul with their daughter Ki-jung and son Ki-woo, facing constant financial struggle and low-wage jobs. When a well-meaning friend named Min-hyuk leaves behind a scholar's rock as a symbol of possible wealth, the Kim family dares to dream bigger. With Min-hyuk abroad for studies, Ki-woo lies about his qualifications to take over his job as an English tutor for Da-hye, the daughter of the wealthy but naïve Park family. Ki-jung helps forge a certificate for him, and Ki-woo, using the alias "Kevin," lands a position with Mrs. Park after a successful first lesson.
The Kim scheme expands rapidly: Ki-woo recommends Ki-jung, who is actually herself, as an art therapist for the Parks' young son, Da-song, who has been shaken by a disturbing sight in the house. By manipulating events, Ki-jung also helps frame Yoon, the Parks' driver, for a supposed incident in the car, paving the way for Ki-taek to be hired as his replacement. They exploit the Parks’ peach allergy to manufacture a claim that Moon-gwang, the longtime housekeeper, has tuberculosis, which allows Chung-sook to take her place. Meanwhile, Ki-woo begins a covert romance with Da-hye, complicating the family dynamics as the deception deepens.
Everything changes when the Parks go on a camping trip. The Kims revel in the luxury of the Park residence, but their world begins to unravel when Moon-gwang returns, claiming to have forgotten something and slipping into a hidden underground bunker built by the house’s architect and kept secret from the Parks. There, Moon-gwang pleads for her husband Geun-sae to remain, as he hides from loan sharks. The truth about the bunker and Geun-sae’s presence is exposed when the four Kim family members, who have been listening in, are discovered, and Moon-gwang films them while threatening to reveal their deception.
The Parks cut their trip short due to a sudden, heavy rainstorm, forcing the Kims to destroy all evidence of their ruse and subdue Moon-gwang and Geun-sae in the bunker, though Moon-gwang is injured in the struggle. The Kim family escapes, but the torrential downpour floods their own basement, driving them to seek shelter in a gymnasium with other evacuees.
The next day, Mrs. Park hosts an impromptu garden party for Da-song’s birthday. The elder Kim siblings assist, while the younger ones are invited as guests. Ki-woo re-enters the bunker with the scholar’s rock, and the mood shifts from celebration to danger. Moon-gwang is already dead from the earlier injuries, and Geun-sae attacks Ki-woo with the rock, leaving him unconscious. Geun-sae then reappears and stabs Ki-jung in front of the stunned party guests as Da-song faints at the sight of what he believes is a ghost. Chung-sook defends herself and kills Geun-sae with a barbecue skewer, but the chaos continues as Mr. Park orders Ki-taek to drive Da-song to the hospital. In a final act of fury and frustration at the Parks’ oblivious privilege, Ki-taek stabs Mr. Park and flees.
In the weeks that follow, Ki-woo is recovering from brain surgery, while Ki-woo and Chung-sook are convicted of fraud and placed on probation. Ki-jung has died from her injuries, and Ki-taek has disappeared, now a wanted fugitive. Geun-sae is presumed to be a homeless man, and the exact motives behind the killings remain unresolved. Ki-woo spies on the Parks’ former home, now inhabited by a foreign family, and notices a Morse-code message visible from outside. Hidden in the bunker, Ki-taek sends daily messages and keeps Moon-gwang buried in the garden, hoping Ki-woo will someday see them. Back in the semi-basement, Ki-woo writes a letter to his father, Ki-taek, vowing that he will earn enough money one day to buy the house and win his freedom.
