Scootaloo is an orphan living in Canterlot after having been adopted by Rainbow Dash and her family. She's been plagued by nightmares of the night her parents were killed and of a monster that may have been responsible for it.
When the monsters start to re-appear she gets recruited by a mysterious government organization to fight them. Now she has to work to save the city and everyone in it from the Order, and she may not have to do it alone.
Henshin!
Obviously this is a crossover with the long running franchise Kamen Rider. We'll get into what that means much later.
Image made by RamenDriver.