Brigadoon is the story of Marin Asagi, a 13-year-old orphan, with an extraordinary problem. The planet Brigadoon has appeared in the sky and Marin is attacked by weird mecha called Monomakia. However just when Marin is about to lose all hope of surviving the incessant attacks from mecha-like weapons, in an old shrine, she finds an ampoule that contains a Gun-Swordsman Melan Blue - an angular blue mecha who swears to defend her.