While she was looking for material to send to princess Celestia, Twilight finds a book! Well, it's not really a book, more a "Alternate universe that somehow got onto the shelf of a small Library in Ponyville". By accident, Twilight falls into it and starts her amazing journey trough the music of Pink Floyd...

(Yes you read that last bit correctly, and no, don't ask me how I got the idea. Let's just say the album Ummagumma Live! and my cat were involved)