Once upon a time, there was a perfect girl named Mary Sue. She was so perfect, in fact, that the author is allowed to start her perfect story with a beginning as lame as "once upon a time."
Mary Sue was the smartest, most beautiful, funniest, most powerful witch on the planet. Only she did not know this. You see, she went to a muggle school in America. One day, when she was sixteen, she got her letter to Hogwarts. No one found it strange that she hadn't started going to magic school when she was eleven, like everyone else. Also, no one found it strange that she was going to school in Great Britain even though she lived in the United States.
Oh by the way Mary Sue is an orphan but that somehow does not really affect her except for the fact that it gives her a "mysterious" past.
So Mary Sue went to Hogwarts and immediately befriended Harry, Hermione, and Ron on the train ride. She told them her story and they thought it was incredibly sad that her parents died. Hermione did not want Mary Sue to have to start out as a first-year at age 16 so she gave her a quick overview of the past five years. Of course Mary Sue caught on right away. She even seemed to know some spells that no one had told her about. She got all of them on her first try, even the patronus charm.
When she got to Hogwarts she was so special that the sorting hat could not sort her. He said she was so smart and brave that she belonged in both Ravenclaw and Gryffindor. Of course, she did not belong in Slytherin because she is so wonderful and nice, or Hufflepuff because everyone knows that is just the house of leftovers. So, Dumbledore created a whole new house for her called Gryffinclaw. Of course she stayed at the Gryffindor common room because that is where all the main characters are.
After dinner McGonagal called her to her office. McGonagal immediately loved Mary Sue. So much in fact that she gave a billion trillion points to Gryffindor for no apparent reason. Also, McGonagal somehow magically knew about how Mary Sue was so smart that she had already learned everything, so she was allowed to skip to sixth year.
In potions class, Snape loved her and her potions skills, which far surpassed Lily's and Snape's and Hermione's combined. She made such a wonderful potion without even using a book. It cured all diseases in the world and it was not too expensive to make so she became famous. Snape awarded Gryffindor another billion trillion points.
All of the other teachers also loved her and so did the students. Every boy in the whole school dumped their girlfriends so they could try to go out with her but of course no one but Harry Potter, who she liked a lot, was good enough for her. Even Draco Malfoy was in love with the perfect, beautiful, smart Mary Sue.
Mary Sue showed up to quiddich tryouts. When she asked what quiddich was everyone laughed, but she showed them. She was such an amazing player that Harry decided she should be seeker instead of him. Meanwhile he switched to chaser and kicked Katie off the team, who no one really ever noticed anyways.
Of course by then Mary Sue had become the most popular student in the school, not to mention more famous that Harry in the wizarding world. She was allowed to join the order of the phoenix even though underage witches are technically not allowed. She became a very valued member and soon ruled the whole club. Not to mention she was very close to Dumbledore.
Later that year there was a Christmas ball, even though Hogwarts does not have yearly balls, only the occasional Yule ball. Harry asked Mary Sue to the ball and of course she said yes. They fell deeply in love and Harry forgot all about what's-her-name, Ron's sister.
Suddenly Voldemort attacked Hogwarts. Mary Sue knew exactly what to do and killed Voldemort with the magical destroy-all-evil, Mary-Sue-spell. Even though there were still horcruxes, that didn't matter because Mary Sue's spell was so powerful.
Harry and Mary got married and lived happily ever after. Meanwhile nothing happened to any other characters because Mary is the only important person now. The end!