After messing up a trick, Rainbow Dash plummets into Applejack, who was in the wrong place at the right time. The Main Six, as well as Spike, find themselves puzzled by the appearance of an odd, red sticky liquid, seeping from a slit in Applejack's leg.

Can they find out what this stuff is or where it came from? Can Pinkie prevent it from taking over the world and destroying everypony?