Yu Ijin, whose story is also widely known as *Teenage Mercenary*, was only eight years old when a plane crash left him as the sole survivor in a foreign land, forced to become a lethal child mercenary just to stay alive. After ten grueling years of combat, he is finally reunited with his family in South Korea, trading the blood-soaked battlefields for the seemingly quiet halls of a local high school. However, Ijin quickly discovers that the civilian world has its own set of dangers when he finds his younger sister being targeted by ruthless bullies. The transition from a professional killer to a protective older brother is immediate and explosive, as he realizes that his specialized skills are the only thing standing between his family and those who wish to do them harm.

Balancing the mundane responsibilities of a student with the violent instincts of a soldier becomes a haunting internal struggle as Ijin fights to keep his dark history hidden from his loved ones. The external pressure intensifies when ghosts from his mercenary past begin to surface, bringing elite rivals and shadow organizations to his doorstep and threatening the fragile peace he has worked so hard to establish. This collision of worlds forces Ijin into a desperate game of strategy and strength where the stakes are no longer just survival, but the sanctity of his home. Despite the rising tide of enemies and the heavy weight of his trauma, Ijin remains driven by a singular, unwavering dream: to protect his grandfather and sister at any cost and finally secure the ordinary, peaceful life he was denied for a decade.