An alien space craft crashes into Graviton City, wiping out the entire population and leaving a massive crater where the city is rebuilt. Students A-ko Magami, a perky, fun-loving red-haired, sailor-suited teenage girl, and her best friend C-ko Kotobuki, a bubbly, carefree optimist, enter a new year of school as transfer students at the all-girls Graviton High School. Although A-ko possesses superhuman speed and strength, she considers herself an average teenager. She mostly worries about getting to school on time, due to her habit of chronically oversleeping her alarm clock each morning. The pair catch the unwanted attention of B-ko Daitokuji, a rich, snobbish, spoiled, and brilliant fellow student.