After one of Bowser's schemes actually winds up being significantly different from all his other attempts at conquest, and unsurprisingly, fails miserably, he and the Mario Bros. are stuck in Equestria. Of course it wouldn't be a MLP fanfic without the usual "I'm going to Ponyville!" cliche, so that's just where the Mario Bros. happen to end up.
Did I mention a good portion of all the power-ups that Mario and Luigi use were also sucked into Equestria? That's right! Watch the hilarity ensue as various ponies encounter various seemingly impossible power-ups, that give them even more impossible abilities! Wanna see what happens when Pinkie Pie gets here hands on a Starman? So do I, but I geuss we'll all just have to live with whatever my imagination can come up with!
I highly recommend that that you watch the Italian Friendship is Magic Opening before you start reading this. (Yes, because Mario is Italian, aren't you a smart one?)
Awesome Mario pony image by this person: http://sklavenbrause.deviantart.com/
[WARNING: MAY OR MAY NOT CONTAIN EXCESSIVELY STUPID LEVELS OF RANDOM!]