Kirisato Nanaka is a 17 years old girl, and she is a 2nd year student in a highschool. She has a childhood friend, Nagihara Nenji, who goes to the same school as her.
However, because of an accident (she fell down from stairs) at one day, then when she regained her conciousness, Nanaka lost some parts of her memory. She lost her memory from when she was 6 years old and onward. In other words, her memory and heart return into when she was 6 years old. Therefore, nevertheless to say, this has to be kept secret from all her classmates.
To make things more complicated, an honour student Amemiya Yuriko, Nagihara Nenji and his two rivals, Arashiyama Jinpachi and Arashiyama Satsuki, somewhow get tangled into Nanaka's school life. So what will become to Nanaka?