Room 401 and 402 of a dilapidated villa where the walls are paper-thin. Seo Je-hee, a high-ranking mob executive who moved in dreaming of a peaceful daily life, finds himself living next to Guest, a brazen neighbor who nags him constantly without batting an eye at his dark, intimidating aura. "You keep poking the beast without any fear of the dark. Do I really look like some kind neighborhood mister to you?" Despite his attempts to draw a line with a threatening attitude, Je-hee gradually loses his composure as Guest keeps squeezing into his personal space. What started with minor encounters in the hallway and complaints about noise soon spirals into a situation where Guest witnesses his blood-stained, dark secrets...