The abandoned tree house was something to poke Louis' curiosity. His secret is kept, until he finds someone else trespassing in his tree house.
or the au where louis is eight and he has tantrums all the time and runs away but one time he runs somewhere and finds a treehouse, then goes back to find harry, then they both share and grow up and fall in love.
It is really quite emotional. Very dark points to be careful of and make sure you read the tags and warnings. It is very sweet when it has to be. Will forever be in my mind.