Orphaned at eight when a plane crash left him the lone survivor in a foreign country, Ijin Yu stays alive by becoming a feared child mercenary with deadly skill in combat and marksmanship. Ten years later, a chance meeting with South Korean soldiers leads him back to what remains of his family: his little sister Dayun and a kindhearted grandfather. Back home in Korea, where life seems safe and peaceful, Ijin faces a new kind of test, his final year of high school. Bullies and new friends alike are unfamiliar ground, and every classroom conflict calls for fresh tactics. When his violent past starts to catch up with him, he must rely on his battlefield experience to protect the people he loves.