Twilight is doing research for Princess Celestia on a new type of pony called a Fang Hoof. They're kinda like vampires, minus the sparkles. On one of Twilights adventures to meet a Fang Hoof, RD falls off a cliff, mistakenly meeting two of them. To save her life, Rainbow Dash is turned into one, and now must live her life as one. Unless they find the cure/