Sonic and Shadow are happily living in Equestria among the various kinds of ponies. It took them some time to adjust to the change in scenery, but they used their natural speed skills to their advantage and managed to afford a nice house from racing. They think they have it made in the shade when a mysterious letter in the mail shatters their peace and threatens their future in Ponyville.

There was no name, no stamp, no handwriting, no prints of any sort. Whoever sent it had been very careful in not getting traced, but what they wanted was clear: the assassination of Princess Celestia and Princess Luna, sisters and co-rulers of Equestria, and there would be dire consequences if they failed or refused.

The worst part? Sonic and Shadow don't know if they can say no.