The Doctor, Amy and Rory walk into the middle of a Western town and say they've come from outer space. As soon as the Doctor introduces himself, the townsfolk grab him, drag him to the outskirts of the town and offer him to a SteampunkCyborg named the Gunslinger. Isaac, the marshal, intervenes and scolds the townsfolk for going against what they already know: the alien doctor that the Gunslinger is most likely looking for is a wholly different man, safely locked up in the marshal's cell.This (equally Steampunk) crash-landed alien, a gentleman scientist named Kahler-Jex, arrived in the town a while ago and had already settled in quite well. Isaac happily acts as his friend and protector. Mercy is a town of second chances, where people have renounced hatred, xenophobia and mob mentality. Jex befriended the townsfolk and even provided the town with electricity — albeit 10 years before its invention on Earth — from his crashed ship. But with the Gunslinger now preventing people from entering and leaving the town, it'll only be a short matter of time before everyone starves.The Doctor thinks this is going to be a pretty straightforward adventure: distract the Gunslinger, evacuate the townsfolk into the TARDIS, fix the crashed ship and celebrate afterwards. He befriends a horse named Susan and goes off to investigate. However, the Gunslinger turns out to be a very Gentle Giant whose programming blocks him from killing innocents unless it's absolutely necessary. Meaning that Kahler-Jex is not exactly innocent at all. In fact, Jex is a war criminal, a scientist who turned innocents into Cyborgs against their will and used them to commit genocide in an interstellar war. The Doctor rushes back to town and goes into a violent rage against Jex, knowing full well that he's Not So Different himself. They both have commited genocide in the name of ending a war, and they've both killed people directly to save the lives of others. All the Gunslinger wants is revenge for having his life taken away from him. The Doctor, blind with anger — more at what Jex reminds him of than at Jex himself — literally throws Jex in front of the Gunslinger and, when that isn't quick enough, pulls a gun and debates whether or not to put a bullet through Jex' head on the spot. Amy brings him back to reality. As the Doctor has a little bit of a Heroic BSOD, the Gunslinger fires, and Isaac makes a Heroic Sacrifice to catch the blast. He dies in the Doctor's arms and hands him the marshal's star.Back in his cell, Jex bitterly calls the Doctor out on his frustration at Jex not being a clear-cut villain. Jex is a genuinely nice person who just wanted to save his people and end the war, and his religious beliefs dictate that he'll have to carry the full burden of his actions after death (quite literally). Meanwhile, the townsfolk have formed a frightened mob and have gathered outside the sheriff's office to come get Jex... using force, if they have to. They just want to protect the town's children and stop living in fear. The Doctor brings them back to reality with much the same arguments Amy used on him earlier that day, and tells them that he'll do everything he can to prevent them from having to live as murderers.In the end, the Doctor stages a distraction and allows Jex to escape into the desert. Jex knows that the Gunslinger will follow him no matter where he goes, and since he doesn't want the lives of more innocent townspeople on any planet on his conscience, he commits Heroic Suicide. The Gunslinger, realising that he's still an immortal monster and that he can never go back home to his normal life again, heads off to do much the same. However, the Doctor convices him to stay in the desert as the town's eternal protector.Once again, Amy and Rory decide to take a break from the Doctor for a while, and they head back home.
Artistic License - Gun Safety: Averted; when inexperienced shooter Amy waves around the revolver she's holding, it discharges twice within ten seconds. Also played straight, as the gun Amy uses is a single action revolver, so she would have had to re-cock the hammer every time she fired. You'd have thought she'd catch on...
As You Know: Isaac tells Rory the details of their plan to distract the Gunslinger so the Doctor can get the TARDIS and save the day.
Isaac: We wait here until the Doctor comes to pick us up in your ship.
The Atoner: Jex is helping the people of the town as a way to atone for creating the Gunslinger. The Gunslinger himself decides to become Mercy's protector, once his purpose is fulfilled.
The Kahler religion actually has this as a core belief, where before they can enter the afterlife, they have to climb a mountain, carrying the souls of those they have wronged in their life.
Kahler-Jex: Do you think I am unaffected by what I did? That I don't hear them screaming every time I close my eyes? It would be so much simpler if I was just one thing, wouldn't it? The mad scientist who made that killing machine or the physician who dedicated his life serving this town. The fact that I am both bewilders you. The Doctor: Oh I know excactly what you are. And I see this reformation for what it truly is. You commited an atrocity and chosed this as your punishment. Don't get me wrong, good choice — civilized hours, lots of adulation, nice weather but, but it doesn't work like that! You don't get to decide when and how your debt is paid!
The episode also opens with a monologue about a "guardian angel who fell from the stars". It turns out to be the Gunslinger, not the Doctor.
Bald of Evil: Possibly invoked. When the 18-year-old cowboy is threatening the Doctor, his bangs aren't visible under his hat. When they depart on friendly terms, they are.
Batman Grabs a Gun: What made it even more surprising is that The Doctor was quick to resort to threatening with a revolver much sooner than expected.
Berserk Button: Kahler-Jex pushes the Doctor's, by playing up how he was forced to maim and kill hundreds to end his war, much like the Doctor, who eradicated his entire civilization and countless others to end the Time War.
And just to twist the knife even further, it's extremely likely Jax did this unintentionally.
Better to Die than Be Killed: Played with; Jex chooses to commit suicide because he doesn't want the Gunslinger to have any more deaths on his conscience.
Black Vikings: Set just after the American Civil War, the local preacher being black is possible, but unlikely.
Though it may be a shout-out to 'This town was named Mercy for a reason'.
Call Back: The Doctor references a number of reoccurring enemies during his mid-episode rant.
Doctor: Today, I honour the victims first. His, the Master's, the Daleks', all the people who have died because of MY MERCY!
Also quite a few similarities to the First Doctor episode "The Gunfighters", including the Doctor being mistaken for another Doctor and immediately arrested because of it.
Deconstruction: Of westerns themselves. The local townsfolk are perfectly normal people instead of Western stereotypes. The barmaid isn't glamorous or raunchy, she's just a woman. The local minister isn't a Bible-thumper, just a sweet man. The marshal wants to help his friends, the town's young hotshot just wants to protect the town's kids. Even the angry mob turns out to be a group of frightened people who are willing to listen to reason.
Framing Device: The episode starts and ends with a woman narrating how her great-grandmother (the little girl seen at certain points in the episode) witnessed the events of the story, and how the Gunslinger came to be the town's sheriff, often thought by outsiders to be a tall tale.
From a Certain Point of View: The Kahler scientists recruited volunteers by claiming that they would make them into the perfect soldiers. They just didn't specify the type of soldiers the volunteers thought they meant.
Funny Aneurysm: The Gunslinger, Kahler-Tek is a cyborg of the same species, the Kahler, as Jex, and so is literally Kahlertechnology.
Kahler Jex: It would be so much simpler if I was just one thing, wouldn't it? The mad scientist who made that killing machine, or the physician who has dedicated his life to serving this town. The fact that I'm both bewilders you!
Amy: Look, Jex may be a criminal, and yeah, kinda creepy -
Kahler-Jex: And still in the room.
Like You Would Really Do It: In universe. The thought of Amy deliberately hurting the Doctor is so ridiculous that when she points a gun at him, he starts laughing. He's right, of course.
Mad Doctor: Subverted. Jex turned a bunch of people into cyborgs and killed several others in the process, but he feels guilt about it and has been trying to atone ever since.
Meaningful Name: The town of Mercy. The morality question that drives the episode is how much mercy Kahler-Jex deserves, and the town provided it to him. It also ties into the page-quote.
Morality Chain: Amy notes that the Doctor tends to slip in his morals without companions to help ground him, which isn't the first time that notion has been brought up.
Never Hurt an Innocent: The first hint of the Gunslinger's real personality is his refusal to endanger an innocent person. He still sticks to this - or, at least, Wouldn't Hurt a Child - after Isaac takes the shot he meant for Jex.
The DOCTOR HIMSELF, of all people, nearly subverts this when he actually points the gun at the other people in the city once the Sheriff tries to stop him from pointing said gun at Jex.
Planet of Hats: The Kahler are apparently known for being excellent engineers.
Queer People Are Funny: Queer Horses Are Perfectly Normal to the Doctor. The local minister is rather confused, though.
Reckless Gun Usage: Amy waves her revolver around and accidentally fires it twice, causing everyone to duck.
Redemption Equals Death: Kahler-Jex pretty much HAS to die in order to save the town, since otherwise the Gunslinger will kill all the citizens of Mercy. But even before he dies, after Isaac saved his life, his last conversation with the Doctor shows he recognizes the depth of his atrocities against the rest of the Kahler race.
Rubber-Forehead Aliens: The only obvious difference between Kahler and humans is the markings on the left side of a Kahler's face. The Doctor exploits this by painting Kahler Jex's marking onto the face of several townspeople, confusing the Gunslinger's auto-targetting program.
A cyborg alien assassin from space chasing after a shifty doctor using advanced technology to help people without knowing about the harm he causes because the cyborg kills people in it's way? Either the plot of this episode or the Doctor Who Book Peacewalker.
Shur Fine Guns: A gun from the Wild West would be a single action revolver, meaning its hammer has to be pulled back manually before it can fire. In the real world, there's no way a gun like that could accidentally discharge twice.
This continues the theme from previous episode, where the Doctor refuses to spare another murderous villain. The difference being the evils committed by Jex were for what he believed to be the greater good, he still felt guilty for doing it, and is trying to repent, Solomon, on the other hand, continued to only be motivated by his own selfish and greedy desires, willing to happily do whatever terrible thing he felt would get him what he wanted.
Not to mention the Doctor's treatment of Jex, throwing him out of the Sheriff's office and shoving him down the street, before sticking a gun in his face. It's like having a more action-orientated Time Lord Victorious.
Oddly enough, Rory, who is usually the first to call out the Doctor in these matters, seems to take his side on this one.