"Is it the worst day of your life?" the black cat asked.
If it is YES, you are invited to this mysterious library.

Thirty-three-year-old Chisa is dumped that day by her fiancé, whom she was supposed to marry in a month. And more, he fell in love with Chisa's best friend. Now, she has no boyfriend, no friends, and no family to talk to as she is estranged from her mother. Chisa is in such a low mood when she meets a black cat in a back alley on her way home. The black cat asks Chisa if she is having the worst day of her life. Chisa, having the worst day of her life, quickly replies, ‘Yes, so what?’ The next thing she knows, she wakes up in a strange retro library. Chisa is panicked, wondering if she is dead or in heaven, but then the librarian appears and explains about the library. That black cat is a ‘messenger’ and asks the people, ‘Is this the worst day of your life?’ and only those who answer ‘yes’ are brought here. This is a place to write a ‘personal history’, and all the inexhaustible books in the library were someone's life. When she finishes writing her book, she can return to the same place she was at the same time when she arrived, but if she doesn't finish writing, she will be stuck here forever. Chisa, who hates writing, is astonished.

Besides Chisa, there is a cheeky girl called Alice, a gentle Madam T who likes tea, a middle-aged man with a scary look in his eyes, a couple who are always together, and a 90-year-old woman who gets senile a little. Chisa faces each of their lives together and sometimes watches them leave the library. She also gets to face her own life.