Aliens called Savages attack Earth, and the only weapon able to fight them is the Hundred. Hayato Kisaragi, a teenage survivor of a Savage attack with the highest known compatibility with the Hundred, enrolls at Little Garden, an elite military academy aboard a seagoing city ship, hoping to become a Slayer. His roommate, Emil Crossford, seems to know him from somewhere, and Hayato senses something strangely familiar about Emil. Right after he arrives, the school's strongest Slayer, the Queen Claire Harvey, challenges him to a duel. As his demanding training goes on, Hayato draws the attention of several female classmates and gets pulled into one incident after another in the fight against the creatures threatening Earth. Expect mecha action, ecchi comedy and a sci-fi military setting.