Mato Kuroi meets Yomi Takanashi on the first day of junior high school, a quiet, introverted girl in whom she quickly takes an interest. Mato tries her hardest to strike up a discussion with Yomi in order to befriend her. At first, she is ignored, but the ice is broken when Yomi notices a decorative blue bird tied to Mato's phone, which is from the book "Li'l Birds At Play," and the two become a deep relationship after discovering they share a mutual passion. The young girls reside as parallel creatures in an other dimension, Mato as BlackRock Shooter and Yomi as Dead Master. What happens in one universe appears to have an influence on the other, and the girls are oblivious of this reality.