A unfinished experiment has escaped from a lab, and is on the loose. Everypony is supposed to be on the lookout. They have been warned that this experiment is dangerous and highly toxic.

Meanwhile, a filly finds herself in Ponyville, and is suffering from amnesia. The only thing that could be a key to her past is a black toxic symbol on her hoof, and she doesn't even know what it means!

Soon, the filly finds herself under the care of Pinkie Pie and her friends, and named Sugar Fluff, at least, until they find her parents. But who is this filly, and where did she come from?