The spring she turns sixteen, Takamiya Naho receives a strange, but detailed letter from herself, ten years in the future. At first she thinks the letter is a prank, but then the things written in the letter actually happen, including the arrival of the new transfer student who sits next to her in class, Naruse Kakeru. The letter reads just like her diary entries, down to the same characters. It is not until two weeks later, when Kakeru shows back up at school, that Naho finishes the letter. In the letter, her twenty-seven year old self tells her sixteen-year old self that her biggest regret is that Kakeru is no longer with them in the future, and asks her to watch him closely.