The Okunoin family has existed in Kamakura since ancient times, but in reality they are the "Kamakura Keshiya," a family that makes a living by erasing people's memories. The Keshiya are a triumvirate of Keshi, who dives into people's memories to stop them, Tome, who brings Keshi's consciousness back to the real world, and Go, who protects Keshi and Tome from outside enemies. However, the Okunoin family has been in a dangerous situation without "Tome" since young Sou became the head of the family. Then, Sou meets Nanoha, an ordinary high school girl in town, and sees the potential of a "Tome" in her.
Nanoha starts "Tome" as a part-time job, but as she dives into people's memories with Sou, she discovers that there are "too many memories to live" in this world...