“Mercy” sounds like a nice name for an old west town doesn’t it? The “Keep Out” sign suggests that this may not be the best place for the Doctor and Co to visit though. They do anyway because, you know, we wouldn’t have an episode if they just went home. There is a Cyborg stalking the lands outside of Mercy and he’s looking for “The Doctor”. Not our beloved man from Gallifrey but ANOTHER alien Doctor. What starts off as a simple mystery about why the Cyborg wants to kill this doctor eventually becomes a battle of morality.