High school student Moon Bin Lee struggles with insomnia caused by a recurring nightmare: he is stranded over a thousand years in the past, pursuing a dark-haired girl into a deep abyss. Soon, the nightmare consumes him, leaving him unable to distinguish whether he is Moon Bin, a Seoul teenager at the end of the second Millennium, or Sa Kyoung Kim, the son of a prominent warrior family in the middle of the first Millennium. As he grapples with determining his true identity and obligations, people from his present-day life assume roles in his historical existence, while he navigates a transmigratory portal across time and space. In the present, his school's kendo club fights to remain in the championships... but in the past, Moon Bin finds himself on the brink of a territorial dispute on the plains of Mongolia ...