Your sister is missing in the world invaded by succubi. Succubi are trying to replace human beings. You cannot inflict physical damages to them. The only way to fight them is to make them feel good. Go on an adventure to find your sister.
Your sister and you lived together since your parents were killed by the succubi. After she told you about the rumor of a fancy party at the castle, she went missing. More succubi came to human habitat to hunt them. You are going on an adventure alone to find your sister.