J is a legendary hitman who is feared in the underworld as "untouchable." One day, while walking on the street, he accidentally collides into a lonely elementary school girl named Megumi. However, because of the accident, they switch bodies.

J is now forced to go through the life of the schoolgirl Megumi, while Megumi is now forced to go through the life of the hitman J. One chaotic and comical situation after another ensues as they go through each other's life.

(Source: MAL News)