影片以2000年3月发生的营团地铁日比谷线中目黑站脱轨相撞事故为背景。事故中遇难的其中一人是少年富久信介。常与他搭乘同一班通勤电车的少女对少年萌生了爱慕之情。但事故突然发生。20年后，一封陌生来信寄到了信介曾训练的大桥拳击馆会长大桥秀行手中。

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?