The story is set on Earth, where alien attacks have become common sights rather than rare phenomena. Nations worldwide have united to develop effective countermeasures against these invasions, but all efforts have failed. In Japan, a secret organization known as GRA (GRAND REFLECT AMUDO) operates under the direct control of the Alien Defense Agency. Its primary goal is to discover a specific countermeasure against alien attacks, with assistance from Dr. Sanada Mio. Dr. Sanada Mio invented a next-generation weapon called "G-on" to repel the aliens. This secret weapon must resonate with the heart of its user (who must be a young girl! You know the standard Japanese anime/manga approach for transformations, bigrazz), transforming into a sword whose power is proportional to the strength of the user's heart. As expected, the weapon and the plan were approved immediately, leading to the establishment of a special school to recruit and train young girls from across the country.