Disclaimer: Harry Potter and Star Trek, and their characters are owned by their respective owners.
Chapter 1:
I found myself in a white room without apparent limit. I saw a familiar face of the actor John de Lancie dressed in a Harry Potter style robe. I said "John de Lancie?" He replied in an amused manner, "Q actually. You are a fan of both Star Trek and Harry Potter and have read several fan fictions of narrators or characters from other genres finding themselves in the Harry Potter universe. It will be amusing to me to see how you'd do."
I worriedly asked "What about ...", Q cut me off with "At some point I'll restore you to your universe where no time will have gone by. You don't have to be obsessed with trying to find a way back. To make it interesting you'll have a complete ability of occlumency so the first legilimens you run into won't acquire your knowledge. You'll also be invulnerable to all mind reading and altering potions, charms, spells, etc, except to the extent that you allow them to work. It will be July 1991, you'll be 11 years old. Yes, you are wondering what the Star Trek tie-in will be. Your name is Tom Picard, born in England. Your family abandoned their home in France during WW2, but have not moved back. The Picard family is a minor wizard family in France, going back to the time of Charlemagne."
"Is this the canon Potter world? Also Star Trek?"
"Of course not, the Star Trek world would have diverged a lot by 1991, instead it is the canon Harry Potter world, with the addition of the Picard family, which results in no visible changes to the world you know. If Renée Picard is born in 1996 she won't cause any timeline changes until 2024. The Harry Potter world, from a muggle perspective, is almost the same as your world, except Star Trek is not found in this world. The only divergences that will occur will be caused by you. This is not to say that there isn't a Star Trek world out there ready to be seen."
Q snapped his finger, and I found myself in an elegant house in front of a mirror. I saw in the mirror, the actor who played a young Picard in the Next Generation episode "Rascals". My memories of Tom Picard flooded in. The man with a slight resemblance to Patrick Stewart was my father Maurice Picard. He said, in voice similar to Patrick Stewart, "Are you quite done fussing with your hair? You won't have it for long, and it is time to go to Diagon Alley for your school shopping."
We got everything needed that I didn't already have. Then we went to get my wand. Ollivander surprised me while I was looking at the many boxes, "Wand hand Mr Picard?" "Right". "Interesting energy. Both of your parents purchased their wands in France. Let's try these black walnut wands". Some sputtering at most. After an array of other unsuccessful wands, I got a strong response, but uncontrolled. "Pine? Maybe Lets try Pine with a Unicorn core. Not a good match. Try this one." It sang, and a light show was performed. "Pine with a Phoenix feather core. 12 and one quarter inches, slightly yielding. I expect to hear interesting things about you Mr. Picard".
When we got back home, I noticed that the merging of my memories with the original Tom's memories was an overlay, but I had the same personality and sense of self. I wondered if Q set it up this way, so there wouldn't be any noticeable differences in the old and new Tom.
The summer was not long, and I practiced from each of the first year classes. Tom's household library helped also, and his parents explained how to avoid trace violations from being discovered.
I wanted to learn about why the Picards didn't still use their ancestral house in France, 46 years after the end of both the Wizarding World War and the muggle Second World War. It seemed to be related to a level of curses on the property that made it not worth the effort to clean it up. However I understood that my purpose in the Harry Potter world was in changing events in Great Britain, not in France, so I dropped this area of research.
What to do with my foreknowledge? Unlike in most stories I had read, I was going to trust Dumbledore. Most stories that I'd read, have the main character worry about being obliviated by Dumbledore, but Tom is immune to that. Then I thought that if that happened I'd pretend it worked, which means that I have to hold back the extent of my abilities. I could reveal my immunity to having my mind read, but I wouldn't reveal my immunity to having my mind changed. Ok, so I'd trust Dumbledore up to a point.
I didn't particularly feel like fighting a troll or Basilisk. I would discover more information, and influence events with information. That means I'd have to work with others, and I didn't see anyone other than Dumbledore I could work with to make changes from Harry Potter canon.