Arisa revolves around Sonoda Arisa and Uehara Tsubasa, twin sisters separated by their parents' divorce. They finally meet again for the first time after three years. Tsubasa is envious but proud of her sweet, popular sister, in comparison to her own school life and Arisa suggests they switch places for a day. Arisa reveals that life isn't as happy for her as Tsubasa thinks and she pushes herself out of the window. Determined to unravel the mystery of her sister's attempted suicide, Tsubasa continues to attend Arisa's school in her place to investigate.