A drama about an old man who is lost in the memories of 1945 Hiroshima after getting an unforgettable birthday letter from his first love. During the Japanese colonial rule era, Moo Gil and Il Ae are childhood friends and each other's first love. Against their wills, both Moo Gil and Il Ae have to leave Hapcheon, South Korea, and go to Hiroshima, Japan. They are separated from each other there. Then in 2019 in South Korea, Moo Gil receives a birthday letter from his first love Il Ae.