One day, Kim Taeyeon receives a letter written to herself from ten years in the future. As Taeyeon reads on, the letter recites the exact events of the following days, including the transfer of a new student into her class named Park Leeteuk.

The Taeyeon from ten years later repeatedly states that she has many regrets, and she wants to fix these by making sure the Taeyeon from the past can make the right decisions—especially regarding Leeteuk. What's more shocking is that she discovers that ten years later, Leeteuk will no longer be with them. Future Taeyeon asks her to watch over him closely.