Chul-soo (Yoo Sung) and his wife Mi-yeon (Eun Jung) live a stagnant married life marked by routine and emotional distance. Their quiet existence is disrupted when a young, attractive couple—Min-seok (Tae Hee) and Ji-young (Ah Young)—moves into the apartment next door. As the two couples begin socializing, mutual physical attraction and curiosity simmer beneath the surface. Driven by boredom and dissatisfaction, Chul-soo and Mi-yeon secretly begin crossing lines with their new neighbors: Chul-soo initiates a clandestine affair with Ji-young, while Mi-yeon engages in a secret sexual relationship with Min-seok. The dual extramarital affairs quickly turn their apartment building into a volatile web of lust, deceit, and domestic undoing.