At the age of eight, Yu Ijin survived a disastrous plane crash in a foreign territory, leaving him as the only survivor. He was forced to become a feared mercenary, perfecting his fighting and shooting skills to become an ultimate predator. But after a decade, when he confronted South Korean soldiers, he discovered that he had a family - a loving grandfather and an adorable younger sister, Dayun.
Returning to his native land, Ijin faces a new challenge; he must adjust to a typical high school life. Every experience, from making new friends to navigating through scheming bullies, is foreign to him. However, as his dark past begins to resurface, Ijin must draw from his past battlefield experience and safeguard his loved ones.