Fujita Kazuyuki is an otaku who turns out to be the one carrying the Millennium Saber within him. That means that he's actually Arest Horn, a legendary hero. Amore Gore, an evil organization of demons, want the Millennium Saber so that they can use it to rule the world.
Suddenly 3 girls, Arisu, Airi, and Akira, appear in Kazuyuki's life. When they transform they become the 3 Musketeers of Arest Horn, and it's there duty to protect Kazuyuki until Arest Horn awakens. So these 3 girls move in with Kazuyuki and his little sister and the hilarity begins.