It was meant to be a friendly visit with the two Royal Changelings, Thorax and Pharynx. But during an attempt at the cutie mark swap spell, Celestia and Luna suddenly find themselves swapping places with the two. Now Thorax and Pharynx are the alicorns, and they're the changelings. To make things even more confusing, somehow no one else knows anything happened. It's up to the royal brothers and royal sisters to fix the problem themselves.