Winter is approaching, and Myne—now an apprentice priestess—must prepare for her stay at the church and the upcoming Dedication Ceremony. However, due to her immense knowledge and extraordinary amount of mana, she has garnered the attention of many dangerous people, who are willing to do anything to get their hands on Myne. To keep her safe, the Head Priest assigns Myne a bodyguard and advises her to be adopted by a noble, a decision that will force her to leave her family behind. As Myne is opposed to the idea, the Head Priest gives her an ultimatum: she can be with her family until she turns 10, but if she is deemed too unstable, she will immediately be dealt with. Placed in a tough position, Myne is uncertain about her future. Despite the twists that may lie ahead, she will do whatever she can to protect those that she loves—even if it means giving up on her dream.

Urano Motosu loves books and has an endless desire to read literature, no matter the subject. She almost fulfills her dream job of becoming a librarian before her life is ended in an accident. As she draws her last breath, she wishes to be able to read more books in her next life. As if fate was listening to her prayer, she wakes up reincarnated as Myne—a frail five-year-old girl living in a medieval era. What immediately comes to her mind is her passion. She tries to find something to read, only to become frustrated by the lack of books at her disposal. Without the printing press, books have to be written and copied by hand, making them very expensive; as such, only a few nobles can afford them—but this won't stop Myne. She will prove that her will to read is unbreakable, and if there are no books around, she will make them herself!

When Myne learns that the Holy Church is in need of mana for their relics, she sees it as her chance to be cured of her life-threatening mana disorder. After seeing their bountiful library, she throws herself headfirst into the Church's grasp and begs to join their order. In exchange for her service and her unusually bountiful supply of mana, Myne is given the blue robes of a noble-born apprentice priestess, despite being a commoner. To Myne, all this talk of mana and nobility is trivial, as she now has access to an unlimited supply of books! As Myne transitions into the next phase of her life in this new world, she soon learns that achieving her dream has come at a heavy cost. Noble society is severe, unforgiving, and fueled by politics and neglect. She must now deal with the class conflict between the noble-born blue robes and the common-born grey robes, the High Priest's attempts to oust her, and constant behavioral issues from her new retainers. With the help of her family, friends, and the enigmatic Head Priest whose loyalties and motives remain unknown, Myne seeks to overcome these obstacles and continue on the path to becoming her ideal self—the ultimate librarian!

Within everyone there exists a side preferably kept hidden, even from close friends. For the smart and popular Kyouko Hori, it's the fact that she has to do all the housework and care for her little brother, Souta, because of her parents' busy work schedules. For the gentle Izumi Miyamura, whom everybody sees as an otaku, it's his nine hidden piercings and large body tattoo. So what happens when they accidentally discover each other's hidden sides? Sharing parts of themselves that they couldn't with anyone else, strong bonds of friendship soon begin to form between Miyamura and Hori, as well as those around them. As their hidden personas start to dissipate, they slowly learn how to open up to others.