Maya and Yuu are a well-matched couple that everyone around them envies. The two have recently started living together and have been nurturing a deep love between them. One day, Yuu's younger sister, Kokoro, comes to Tokyo from her parents' home. Yuu is happy to see her again after a long time, and Maya can't hide her happiness at meeting Yuu's younger sister for the first time. However, Kokoro has a secret plan. "I can't help but want to steal other people's things."Kokoro steadily begins to prepare to seduces her sister's boyfriend, Maya. And the day to act arrives...