Song is Available at: http://deltabrony.bandcamp.com/track/powerpuff-rainboom-crossover-euro-mix

Rainbow Dash performed a Double Rainboom, which she created a portal to another world. She ended up in Townsville where she met the Powerpuff Girls for the first time before Pinkie brought her back to Equestria. The girls went back to their home and told the professer that there is a pony that they met in Townsville and he told them that is Rainbow Dash from Ponyville. The girls wanted to go there and see the other ponies, so the Professer created a portal machine to transport the girls to Equestria. Looks like the Mane 6 is going to be shocked about this when they meet the Powerpuff Girls.

MLP by Hasbro
PPG by Cartoon Network