Mono is a lonely girl who, despite belonging to a family of renowned opera stars, has quietly lived her whole life neglected up in the attic of the theater.

The only person who would ever spend time with her was her kind older sister, who would always visit to play and teach her how to sing and dance... Until one day, she was gone.

Mono eventually learned that her beloved older sister was being held in a containment facility known as a “Libratarium,” an establishment built to isolate those who have contracted the terrifying disease known as “Paper Mortis.”
Paper Mortis is an ominous illness that transforms animals into monsters and people into picture books.
The disease has become so dire that the Libratarium that Mono’s sister was taken to has been abandoned, with all of the patients and monsters alike still inside.

Unable to stay still, Mono journeys to the Libratarium to rescue her sister before it’s too late.

“Wait for me, Sister. I will help you, by all odds.”

Shortly upon entering, she meets the legendary monster named Mär and makes a contract with him in order to search the tragic stories of the picture books within.

“...All right. Together we go, then, Mär. But you have to keep me safe in return, okay?”
“Yeah, ’course I will. Let’s you and me play nice as best we can, human.”

Even though she knows that Mär is plotting his revenge against humanity, Mono is determined to work alongside him in order to save the only person who has ever loved her.