Imagine waking up one day to find that the sky around has been engulfed by an unidentifiable mechanical presence. Imagine that this new sky opens up and releases a Monomakia, a living weapon, whose sole purpose is to end your life. That's exactly what happens to 13-year-old Marin Asagi, an orphaned girl living in an alternate version of Tokyo in the 1970's. Young Marin's only chance to survive rests with Melan Blue, the Gun-Swordsman, who is destined to protect her. With arms made of weapons, the power of flight, and retractable body armor, Melan is a powerful bodyguard indeed.

Who is sending the Monamakia? Why do they want Marin dead? Who sent Melan to protect her? What is the secret of Brigadoon? With her new protector by her side Marin sets out across two worlds to discover the answers.