Everyone has regrets in life. So who wouldn't take the chance to change the past if given the opportunity? When sixteen-year-old Takamiya Naho receives a mysterious letter, claiming to be from her twenty-seven-year-old self, her life is suddenly thrown into flux. The letter informs her that a new transfer student by the name of Naruse Kakeru will be joining her class, and to keep her eye on him. What is Naho to make of the letter's contents and its cryptic warning?