Goryeo prince Jin Yuseong is a martial arts hero, but after losing his comrades, his loneliness drives him to walk through the palace's magic gate for a fresh start. Arriving in modern-day Seoul, he becomes an unexpected hero in a gate break mission, drawing the attention of the Safe Gate organization, who protect civilians from the gates. Eager to escape recruitment into SG, Yuseong disguises as the high school nephew of a former comrade, but will this be enough to outrun his fighter past?