For years, BTS believed Kim Seokjin was an only child—until a mysterious young woman appears at his military enlistment ceremony. To everyone’s shock, she is revealed to be Jin’s younger sister, Y/N, whom he has spent years protecting while she studied at Columbia University in New York. While the members are eager to get to know the sister Jin never talks about, Taehyung is left stunned for an entirely different reason: Y/N is the stranger he shared an unforgettable night with just days earlier. Now forced into each other’s lives, both must navigate secrets, growing feelings, and the terrifying possibility of Jin finding out.