The Doctor has been living in Equestria for a good while now, well suited to it. But when another Doctor appears, from an alternate dimension, things get complicated. Specially since this new Doctor brought the biggest threat to Equestria with him.
Sol- More famously known as Nightmare Sol- has followed him through the dimensional travel. It seems that there's more than meets the eye about the strange dimension he's come from.
There are no famous Doctor Whooves companions in this story. All companions are OCs. Even the Doctors are kind of... Different. Regenerated. Not the design from the show.