Can somebody sum it up for me?
A guy IRL finds filly Rainbow Dash in a cardboard box and raises her (nothing NSFW). Then when she's grown up Princess Celestia and the other main ponies show up and it appears that RD had gotten there because of some spell that cast her into our dimension.
Then they all go back into their dimension, and the guy whose name is never known remembers his time with Dash.
That's a very short version, but it really is worth reading. It was the first fanfic I ever fully read.