Twin sisters, Arisa and Tsubasa, are separated by their parents' divorce. The twins secretly exchange letters during their separation. Finally reunited after three years, Arisa asks Tsubasa to pretend to be her and spend a day in her school just for fun. At school everyone seems very nice to Tsubasa (who is pretending to be Arisa) and everything seemed normal... until she finds a strange note in her sister's shoe locker. When Tsubasa hands over the letter to Arisa, Arisa tries to kill herself. Arisa survives but remains in a coma. The letter read, "Arisa is a traitor." Determined to unravel the mystery behind the act and the letter, Tsubasa decides to investigate...