One day, Naho Takamiya receives a letter written to herself from ten
years in the future. As Naho reads on, the letter recites the exact
events of the day, including the transfer of a new student into her
class named Kakeru Naruse. The Naho from ten years later repeatedly
states that she has many regrets, and she wants to fix these by making
sure the Naho from the past can make the right decisions, especially
regarding Kakeru. What's more shocking is that she discovers that ten
years later, Kakeru will no longer be with them. Future Naho asks her to
watch over him closely.