Mato Kuroi befriends her quiet classmate Yomi Takanashi when they start junior high, and the two grow close, with Mato giving Yomi a blue star phone charm as a token. When the next year separates them into different classes and Mato drifts toward a new friend, Yomi feels increasingly alone. Their fraying bond plays out alongside a wordless parallel world where an armed girl called Black Rock Shooter clashes with the horned Dead Master.