The first part of Date A Bullet.

Hibiki Higoromo, better known as, Empty, is an amnesiac young girl. She wakes up in the neighboring world, where she encounters Tokisaki Kurumi. In order to kill the mysterious girl, Kurumi leads Empty to a school. When arriving at the school, a number of girls, known as semi-spirits, gather together. Now then, a new battle begins.

(Source: Wikipedia)