After falling through a gate into another world, Lee Kangjun spends 300 years conquering continents, slaying dragons, and becoming an emperor. When he finally returns to Earth, his sister hands him a daughter he never knew he had, saying the mother died in childbirth. Though treated like a delusional man, Kangjun decides to use all his power, not for conquest this time, but to ensure his daughter Seol-yi lives a happy life.