The world is divided into two kingdoms: the Moon Kingdom, inhabited by humans, and the Sun Kingdom, inhabited by demons. Estelle, a knight of the Moon Kingdom, loses her family in a demon attack, and is sentenced to a life of imprisonment after falling in love with a strange demon girl. To survive, she must travel to a tower in the north. A timid girl knight, a slave boy, and a ghostly demon. The story of three people who embark on an adventure to find their lives.