A man named Oh Dae-su leads a fairly unremarkable life until one day, without explanation, he's abducted and imprisoned in a cell that closely resembles a rundown hotel room. For the next fifteen years, his only contact with the outside world is through the television in his room and the occasional interactions with his captor's minions. During his prolonged confinement, Oh Dae-su is framed for the murder of his wife, and his daughter is taken away to be adopted. While imprisoned, he undergoes a transformation, using his anger and desperate need for vengeance to fuel his resolve to escape and find those responsible for his misery. He trains relentlessly, preparing himself for the day he can reclaim his freedom and seek revenge.

Upon his unexpected release, Oh Dae-su enters a bewildering reality, where he meets a young chef named Mi-do, who takes pity on his disoriented state and offers him help. Determined to find the architect of his suffering and to understand why he was imprisoned, he follows a series of cryptic clues and uses Mi-do's assistance to navigate this complex path. However, the deeper he investigates, the more puzzling the mystery becomes, revealing layers of deception and a past that ties back to his old school days. With each step closer to the truth, the web of secrets grows more tangled, and the implications of his confinement more horrific than he could have ever imagined.