On the corner of a modern town, there is a private library called the "Clover Library". During the day, it seems like an ordinary library. However, when night comes, it becomes a strange library where the books take on human forms.
The cookbook, Natsume. The picture book, Riku. The book of paintings, Shikimi. The technical book, Itsuki. The book that's the latest craze, Aoi. Aikawa Chihiro, the protagonist, became the library's live-in caretaker. While somewhat inept, she continued to manage the library.
One day... The library's pet cat, Kuu, brought a single key to Chihiro.
"I wonder where this key is for? When it's night, I'll try asking everyone."
Now, on the very top floor of Clover Library, the so-called "forbidden door" will be opened.
[Translation of the official website's story summary.]