Ever since the TARDIS dumped him in Ponyville, the "ponified" Captain Jack Harness has been slightly scared of Sparkler. The unicorn is crazily defensive of her mother and sister, and he has no idea why.
Meanwhile, Sparkler wonders why that strange pegasus, Jack Harness is in Ponyville. How does he know the Doctor? And why does he have so much trouble flying?
Jack doesn't realize that Sparkler has endured quite a bit of teasing and loneliness. And Sparkler has no idea that Jack wasn't born a pony– or anything remotely like a pony, in fact.