Harry walks in and find two Hermiones. What is he to do? I know, not a very good summary, but the story is much better, I promise.
Harry and Hermione end up growing up fast, right around the same time. Suddenly, Harry is a total hunky stud and Hermione is noticing. Meanwhile, Hermione has become a curvaceous, buxom young beauty and Harry has DEFINITELY noticed that.
Hermione goes back in time to play with herself.
Hermione uses her Time Turner and Polyjuice Potion for a little fun, but things get a little out of hand.