Twin sisters, Arisa and Tsubasa, are separated by their parent's divorce. When Tsubasa pretends to be Arisa on day at school, she receives a mysterious letter that prompts Arisa to attempt to kill herself by jumping out of a window. The letter read, 'Arisa is a traitor.' After her sister's suicide attempt, Tsubasa is determined to unravel the mystery behind the letter and her sister's suicide attempt so Tsubasa continues to go to school as Arisa. Will she find out who sent the note and why it had such an effect on Arisa?