This is not the world you know. There roams a girl with jet-black hair, ebony clothes, and a strikingly blue, glowing eye. Her name is Black Rock Shooter. With a huge cannon in hand, she throws herself into a fierce battle...Meanwhile, in another world, middle-school student Kuroi Mato is drawn to Takanashi Yomi, her classmate with an unusual family name. Mato does her best to talk to Yomi, who just will not open up. Koutari Yuu belittles her best friend Mato's efforts but at the same time, supports her. Mato also finds herself surrounded by other colourful characters at school, like the eccentric school counsellor, Irino Saya, and the hot-blooded captain of her club club, Kohata Arata.Suddenly, one day, the window of opportunity opens. After Mato sees that Yomi has her beloved picture book, "Li'l Birds at Play", they strike up a conversation and Mato is invited to come over to Yomi's house. There, they gradually get acquainted, until a girl in a wheelchair appears. This is Yomi's best friend, Izuriha Kagari...Two worlds. Mato and Black Rock Shooter. This is the story of their "agony".