While the investigating an interdimensional rift, The Doctor finds himself in a strange dimension of talking ponies and discovers he has regenerated into one of these ponies. he begrudgingly enlists the help of a clumsy pegasus named Derpy to assist him in getting used to this new world as well as possibly finding a way home. Meanwhile, the city of Fillydelphia is under quarantine because of a massive infestation of parasprites. Can The Doctor and Derpy stop the Parasprites before the whole city is consumed?

(This story Takes place after "The Angels Take Manhattan" In Doctor Who and sometime after "Swarm of the Century" In My Little Pony.)