Marin, a cheerful junior high student with a loving adoptive family, faces a sudden upheaval when a strange mirage appears in the sky worldwide. Menacing androids known as Monomakia emerge from this phenomenon above Brigadoon, targeting Marin for unknown reasons. In her quest to escape, Marin stumbles upon a mystical blue bottle in a shrine, unleashing a sword-wielding, gun-toting alien protector named Melan Blue. Together, they must defend Earth, navigate family turmoil, societal biases at school, and evade the authorities. Through their journey, they uncover Marin's past, decipher Melan's enigmatic purpose, and cultivate a bond of trust while encountering a diverse array of allies and adversaries in the backdrop of 1969 Japan.