Mono is a lonely girl abandoned by her parents despite being the descendant of famous opera singers. She lives quietly in the attic of a theater, worked like a servant day after day — until her beloved older sister, the one person who ever showed her kindness, goes missing. The trail leads to Ishokan, an isolation facility for those afflicted with Shinigami Disease — an illness that turns living beings into picture books that then emerge as monsters. Unable to sit back and do nothing, Mono visits Ishokan and forms a contract with the legendary monster Mel to protect her as she searches the monster-filled halls and the inner worlds of the afflicted girls, determined to save the only family who ever loved her.