Saki and her big brother Keisuke have always been on their own since they were little and had to support each other since their father was never home. Because of this, a deeper bond developed between the two. When Keisuke lost the talisman he got from his father one night, his sister suddenly shows a frightening side because she is a succubus - and she can no longer contain her lust, can no longer contain her feelings for her brother and falls upon him that same night. When his classmate Riri Manabe attacks him the next day at school, Keisuke realizes that something is wrong since he lost the talisman. What he doesn't know yet is that he has a higher than average level of mana, which attracts a lot of succubuses. Only his father's talisman could suppress this. But since he lost it, he is now completely helpless against the lust of Saki and Riri.
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