One day, after Cadance and Shining Armor beat King Sombra's attempt to take over the Crystal Empire, they find that the evil king has, in fact, a little unicorn filly, who they take in as their own to raise her. Flurry Heart, the present daughter of Cadance and Shining Armor, accepts her new, adopted sister named Precious Heart, and the two grow up together and love each other as sisters do.

However as Precious grows up, she displays some weird behavior, which eventually leads to the revealing that Sombra has taken control of his daughter and uses her to try once again and take over the Empire he so seeks to conquer. It's up to Flurry to break through to her sister who she loves and bring her back...

(Set in a slightly alternate universe from FiM, also might change the rating to T, if it gets there.)