4 needed to calculate an average
Fifteen-year-old Tsubasa's parents divorced three years ago, separating her from her beloved twin sister Arisa. However, they've communicated secretly with letters and are now meeting with each other for the first time since the event. Arisa is everything Tsubasa is not - she's popular, gentle, has good grades and has a new boyfriend, while Tsubasa can't seem to find girl friends to be cutesy with and has earned a reputation of being a delinquent leader. The two decide to switch places for a day and Tsubasa is enamored with Arisa's school life. But beneath the surface of Arisa's life lies dark secrets, not only about her classmates, but also Arisa herself. Tsubasa must infiltrate her beloved sister's life in order to discover the truth behind shocking events taking place...
Both stories revolve around a person who dies or gets seriously injured, and then an identical look-alike who takes their place and tries to solve the mystery of what happened to the original.