Hermione is older and out of school now. Since she removed herself from her parents’ memories before the war, she set out to find them again…well, just to find her dad. She may have changed her hair and her look somewhat, but she’s surprised when her dad has no idea who she is–he doesn’t recognize her at all. So she does what any horny, taboo witch would do in her place. She seduces him. While he’s fucking her, she removes the memory charm, making him realize that he’s been fucking his daughter this whole time. She knew it was a risky move, but she also had a strong feeling he wouldn’t want to stop. After all, she can always redo the memory charm after it’s over and he’ll never have to deal with the consequences.
