Flash Sentry has been hitting on Twilight Sparkle. As her older brother, Shining Armor can't stand it; he hates Flash Sentry. So, he kicks Flash Sentry's flank to stop him from dating Twilight.
Applejack disguises herself as a stallion to serve the Duke Spike, who sends her to woo the Lady Rarity who ends up falling for Applejack, who is really a mare in love with the Duke.