The manga is about a high school student Yuu Sakagami who, raised separately, is reunited with his mother, Misako, and his sister, Eri, who both become infatuated with him. When Yuu becomes sexually aroused his inner demons take over and he is unable to control his actions, and the women in his family, his English teacher from America, and a female neighbor (who is later revealed to be the boy's aunt), are quick to take advantage of this. The situations and humor are similar to the non-sexual manga, Ranma ½, but nearly always have a sexual focus.
Yuu starts out having slept with his sister Eri, but eventually one night he and both his mother and sister get drunk and they have a threesome. The insanity continues when Yuu gets disgusted with what he is doing and leaves the house, only to be taken in by his English professor, who seduces him. This ends with him swearing to give up sex because of how he gives in to his dark desires.
Next we find Yuu and Eri making wishes for what appears to be New Year's, Yuu wanting to "get it on" with a woman, Miss Sakura. Eri later witnesses said woman having sex with a man, but doesn't tell Yuu. Yuu decides to get his fortune told, and it comes back with these words; "Doom. Death. Defeat. Despair." Also it has a tiny message at the bottom; "sorry, kid".
Next we see an encounter between Yuu and Misako, in which Yuu is eating food made by Misako. Misako is wearing nothing under her apron, and tries to seduce Yuu. Yuu tries in vain to keep from getting aroused, but Misako sits on his lap and that's that. Yuu envisions his "reason" getting run over by a train marked "instinct" and it's off to the races again. Yuu and Misako have sex, after which Yuu looks in horror on what happened and comments that he's going to hell.
Yuu and Eri next arrive home to find their mother passed out after a session of phone sex with her boyfriend. This ends with Eri seducing Yuu again and Yuu giving in once more. Next, Yuu has an encounter with the woman next door, who has the nickname "virgin killer". She mistakes Yuu for a virgin and seduces him using special chocolates. Yuu gives in to instinct again and has sex with her. During her climax it is revealed that the woman next door is actually his aunt. When Yuu gets home, Eri accosts him with an even larger batch of more potent love chocolates.
Yuu then has another encounter with his Mom, Misako, in which she forces his darker side to stay active so she can have sex with him. Yuu has sex with Eri again, in which he violates her against her will and hurts her pretty badly.
Misako talks with Yuu and it is revealed that Yuu's dad had the same problem Yuu does, and that it why he's not around anymore. Misako implores Yuu to keep control saying that it is all a matter of willpower. Yuu ponders this and envisions a battle within his mind between Reason and Instinct, both attractive women in this vision, the actual "battle" being a lesbian scene between the two. Yuu eventually chooses Reason, and he defeats his dark side. He wakes up and Misako tells him he has won, but soon realizes the regular Yuu is a virgin. She teaches him all about sex in a last encounter, and Yuu goes back to being himself. The story ends with Yuu stating how far he is come since that time, and admitting that he and Eri still have sex.