Marin Asagi is a typical junior high school girl with a loving adoptive family. Her life takes a drastic change when a mysterious mirage is seen in the sky above the entire Earth. The mirage is actually another world called Brigadoon (named after a town in the musical of the same name, that only appears at certain times). Soon alien creatures called Monomakia descend from the formation in the sky and hunt down Marin, but she is saved by another Monomakia named Melan Blue, a flying, sword-wielding, gun-slinging alien who becomes her protector. Together Marin and Melan must save the Earth and deal with family crises, school prejudice and the police, and come to an understanding of Marin`s past and Melan`s unexplained mission, as well as learn to trust each other.