In their first year of high school, a girl named Mizumura and a boy named Sakahira fell into a swimming pool and mysteriously switched bodies. They live out each other's lives for 15 years, all through high school, college and beyond, and frequently exchange information about what's going on in their lives. Then, all of a sudden, Mizumura tells Sakahira that she might have found a way for them to switch back. But do they even want to do that at that point?

A woman writes a letter to a young man, leading to a correspondence with his father that unveils the truth of a 24-year-old high school romance, his son's death, and the power of love letters.