Big Macintosh must thwart an evil pony's scheme to cause a major crop disaster.

The song from the fic "The Great Bale Out" by Fairy-Slayer!  http://fairy-slayer.deviantart.com/art/The-Great-Bale-Out-197219507

It is, naturally, a cover of the Perry the Platypus/Agent P theme from "Phineas and Ferb" with Pony-themed lyrics.

Music by me: http://tinyurl.com/whosthisjerk
Lyrics by Fairy-Slayer: http://fairy-slayer.deviantart.com