The episode featured alien time traveller the Doctor (Matt Smith) and his companions Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill) visiting the Wild West, where they encounter a town which is cut off from the rest of the frontier until they hand over Kahler-Jex, an alien doctor, to a cyborg called the Gunslinger.

"A Town Called Mercy" and the previous episode "Dinosaurs on a Spaceship" were the first to enter production for the seventh series.

They find that no one has been able to leave or enter town due to "The Gunslinger", who demands they turn over the doctor, who transpires to be Kahler-Jex, an alien scientist that crashed to Earth years earlier.

The Doctor sneaks out to get the TARDIS but finds Jex's crashed ship and discovers he was one of several scientists that did inhumane experiments to create cyborgs on their war-torn homeworld.

[3] Moffat was keen on putting Matt Smith in a Western setting, who he called one of the last people one would expect to replace Clint Eastwood.

[7] Smith praised guest actors Ben Browder, who he said "[made] a good cowboy" with "that great drawl", and Adrian Scarborough, who he said "steals the whole episode".

[8] Browder was offered the role and gladly accepted; he was aware of the show as his children had watched it, and he also wanted to do a western.

[6] Moffat stated, "We knew from the start we need some serious location shooting for this one, and given the most iconic American setting imaginable, there was only one place to go – Spain.

McGrath also noted a "take-home religious message" in the scene near the end where the town gathers in the church while the Gunslinger and the Doctor face off; he felt that it emphasised the importance of valuing human life.

Club reviewer Keith Phipps noted that the "never-ending struggle between order and chaos" was common in Westerns, and the episode represented this with the question of "what should win out: Lawless revenge or civilized justice?".

[19] Ian Berriman of SFX interpreted the border around Mercy as a metaphor for the Doctor nearly "crossing [the] line" and "[breaking] his own moral code".

[20] In addition, reviewers noted that the episode presented its characters with "shades of grey", rather than black-and-white villains typically seen in the show.

[24] When final consolidated viewers were taken into account the figure rose to 8.42 million, also beating "Asylum of the Daleks" to be the highest rated of the series.

IGN's Matt Risley rated the episode 8.5 out of 10, calling it "a weighty, progressive, sumptuous and entertaining adventure".

He wrote that it was Karen Gillan who "emerged as the real star of the episode", citing Amy's conversation with the Doctor about how travelling alone had affected him.

Club reviewer Keith Phipps gave the episode a B+, enjoying that it spent most of the time discussing the morality issue.

Though he also praised the "well-crafted" scene between the Doctor and Amy, he felt that the episode was "a waste of Gillan and [Arthur] Darvill's talents" as the two did not feature much.

[18] Digital Spy's Morgan Jeffery also gave it four stars, commending the Western atmosphere and the way the Doctor's darkness was handled.

[29] Slant Magazine reviewer Steven Cooper described it as "a very enjoyable episode", though he noted that "the conclusion of the story is a slight let-down after the excellence of what has preceded it" because the issues between the Doctor and Jex were left unresolved.

She found the Doctor holding Jex at gunpoint as "completely uncharacteristic", but interpreted it as foreshadowing Amy and Rory's departure.

She criticised the Gunslinger for neither simply using its presumably advanced targeting systems to kill Jex or making the townspeople leave so that he would not have human shields.

But in a show that usually has a lot of fun reinventing TV tropes, too much of "A Town Called Mercy" feels a bit been-there, done that.