Discord has appeared many times throughout equestrian history and is considered a very dangerous being. The one day when he wrecks anarchy on Ponyville once more. A certain pony finally snaps at him.

The doctor and derpy go back in time to visit old Canterlot. But this is Canterlot before Celestia and Luna were rulers, meaning the most hated one in equestria is currently ruler of equestria. The doctor wants to put a stop to that.

After an engine failure, the TARDIS is falling. The Doctor tries to think of a way to get it starting, but the TARDIS ends up crashing. Derpy survives, but the doctor doesn't. The crash was fatal enough to trigger one thing though. Regeneration.