Marin Asagi is a typical junior high school girl with a loving adoptive family. Her life changes drastically 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 appears only 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.