Dinky has always been different. The most creative and outgoing filly, excelling in all subjects, and her athletic skills are much better than even the pony with the cutie mark of sports! Not to mention two beating hearts.
Dinky finds out that her father was an earth pony, and her mother has no relation to unicorns, yet Sparkler and Dinky both are. She learns so much faster than other children, and asks the most questions. And she is such a curios filly.
Dinky can't help but notice, and she wonders why. She asks her mother, Derpy Hooves, but all she says is that she is "a special filly."
Sparkler doesn't have crazy skills, but she does have two hearts. Derpy starts to worry, and doesn't want to tell Dinky or Sparkler about her father, The Doctor. Derpy doesn't want to endanger Dinky or Sparkler, but when strange things start to happen, Derpy has no choice.