From inmeliora:

Eight years ago, Mio, the princess of the White Kingdom, and Sakuya, the second prince of the Dark Kingdom, became friends when she got lost in the forest and Sakuya helped her. They were friends until one day, three years ago, she was sucked into a hole and went missing, and Sakuya is determined to find her. In the three years since her disappearance, he became friends with her older brother, Rui, the second prince of the White Kingdom, even though their two kingdoms don't get along. One day, while joking around, a hole opened up in the sky and a girl fell out of it. Sakuya had dreamed this would happen, but who is this girl and why did she fall into heaven?!