The Kim family, consisting of father Kim Ki-taek, mother Chung-sook, son Ki-woo, and daughter Ki-jeong, live in a small semi-basement apartment, working low-paying temporary jobs and struggling to make ends meet. Ki-woo's friend Min-hyuk, who is preparing to study abroad, gifts the Kim family with a scholar's rock that is supposed to bring them wealth. Min-hyuk suggests that Ki-woo pose as a university student to take over his job as an English tutor for the wealthy Park family's teenage daughter, Da-hye. Once Ki-woo is hired, the rest of the Kims successively pose as sophisticated skilled workers, unrelated to each other, and integrate themselves into the lives of the Parks: Ki-woo begins a romantic relationship with Da-hye; Ki-jeong poses as an art therapist and is hired to teach their son, Da-song; Ki-taek is hired as a chauffeur after Mr. Park's driver is fired when Ki-jeong frames him for having sex in the car; and Chung-sook is hired to replace the current housekeeper, Moon-gwang, after the Kims exploit her allergy to peaches, claiming that she has tuberculosis.
When the Parks leave on a camping trip, the Kims occupy the mansion, revelling in its luxuries. That night, Moon-gwang returns, claiming that she left something in the basement. She opens a secret bunker, revealing that her husband, Geun-sae, has lived there for years to hide from loan sharks. As she begs Chung-sook to keep their secret, the eavesdropping Kims accidentally stumble into view. Moon-gwang threatens to expose their scam after realizing they are a family, leading to a fight.
The Parks suddenly call, informing Chung-sook that heavy rain has ruined the trip and that they are arriving home shortly. The Kims force Geun-sae and Moon-gwang into the bunker; Moon-gwang is kicked down the stairs by Chung-sook and suffers an eventually fatal head injury. When the Parks return, Mrs. Park tells Chung-sook that her son was traumatized years earlier by seeing a "ghost" emerge from the basement: Geun-sae. With the other three Kims hiding nearby, Mr. Park complains to his wife that while Ki-taek is a competent driver, he smells bad. The three Kims escape the mansion undetected, but find their apartment flooded by sewage water driven by the rain. They spend the night in a local gym, along with hundreds of others driven out by the rain.
The following day, Mrs. Park throws a birthday party for Da-song and invites the staff. Ki-woo returns to the bunker with the scholar's rock but is ambushed by Geun-sae, who hits him over the head with the rock. Geun-sae escapes the bunker and rushes into the party, stabbing Ki-jeong. Geun-sae's sudden appearance triggers Da-song's traumatic memory, causing a seizure. As Ki-taek rushes to help Ki-jeong, Mr. Park yells at him to drive Da-song to the hospital. Ki-taek throws him the car keys, which land under the struggling Chung-sook and Geun-sae. Chung-sook manages to kill Geun-sae with a meat skewer. As Mr. Park retrieves the keys, he recoils from Geun-sae's smell. Ki-taek, witnessing Mr. Park's reaction to the smell, fatally stabs Mr. Park before fleeing.
Weeks later, Ki-woo awakens from a coma and is sentenced, along with Chung-sook, to probation for fraud; Ki-jeong has died from her wound. Geun-sae’s motive and the whereabouts of Ki-taek, who is wanted for Mr. Park's murder, are unknown. While watching the Parks' recently sold mansion, Ki-woo notices a light flickering in Morse code – a SKATS message from Ki-taek, who now lives in the bunker. Ki-woo vows to one day earn enough money to purchase the mansion and free his father.
