Deficit was a pony created accidentally in an alternate universe within another alternate universe. He was sent to Equestria because his creators didn't know what to do with him.

In Equestria, he finds that thousands of unicorns and earth ponies are becoming alicorns and pegasi. He teams up with a few other ponies to stop the seemingly endless transformations, caused by none other than M.A. Larson.

But they soon find that there is more to the picture.