A few years before Twilight Sparkle and her fantastic friends were born, a dimensional rift was known to have opened up very near Ponyville. A number of ponies were lost into the void, and Celestia kept the rift a secret, hidden from sight until Twilight became an alicorn and a princess; now the Six Mane Ponies are tasked with entering the "Human World" on the other end, becoming humanoid themselves and attending high school as a cover. Before they leave, Trixie finds out and is determined to follow, though she quickly finds herself lost. Story follows Trixie's party leading to the Human World and Twilight's party who have already made it there.
Crossover elements dominate on Trixie's end, whereas "OC" involvement dominates on Twilight's end.