Ryou Sato is in a committed relationship, but everything begins to unravel when he becomes involved with his girlfriend's older sister, Kana. What starts as an unexpected encounter gradually develops into a secret affair fueled by temptation, guilt, and emotional conflict. As the forbidden relationship deepens, the two struggle to hide their feelings while risking the trust of those closest to them. A dramatic story exploring infidelity, desire, and the consequences of crossing the line.