The Doctor, Amy and Rory arrive in Mercy, a frontier town in the Old West which boasts electricity a decade too early. Mercy is being terrorised by a murderous cyborg in his search for Kahler-Jex, an alien surgeon who took refuge there after his spaceship crashed in the nearby desert. The townsfolk -- led by their marshal, Isaac -- are determined to safeguard Kahler-Jex, but supplies and morale are beginning to run low. As the Doctor uncovers the sordid history between Kahler-Jex and the cyborg, he begins to realise that, sometimes, the line between victim and monster is very blurry indeed.

Movies set in the American Wild West were popular from the earliest days of the cinema, and never more so than during the Forties and Fifties, in the hands of filmmakers like John Ford and Howard Hawks. The Sixties then saw the rise of low-budget “Spaghetti Western”, typically made by Italian directors in Spain and Italy. Perhaps the most acclaimed Spaghetti Westerns were three movies directed by Sergio Leone and starring Clint Eastwood as the “Man With No Name” -- Per un pugno di dollari (A Fistful Of Dollars, 1964), Per qualche dollaro in più (For A Few Dollars More, 1965) and Il buono, il brutto, il cattivo (The Good, The Bad And The Ugly, 1966). Leone shot the trilogy in the Tabernas Desert, located in the Spanish province of Almería. Wild West-style towns and forts were built for Leone's productions, and they were subsequently maintained as theme parks called Oasys (or “Mini Hollywood”) and Fort Bravo (or “Texas Hollywood”).

By Season Thirty-Three, filming outside the United Kingdom had virtually become an annual tradition for Doctor Who, dating back to 2007. Most recently, cast and crew had visited Utah and Arizona for the Season Thirty-Two premiere, The Impossible Astronaut / Day Of The Moon. For the programme's next overseas excursion, it was decided that the Doctor Who team would travel to Almería, which suggested the inclusion of a story with a Western setting. To develop the adventure, executive producer Steven Moffat turned to Toby Whithouse, who had last contributed Season Thirty-Two's The God Complex. While he had no prior experience writing Westerns, Whithouse was fan of Leone's work; he also admired Deadwood, a mature take on the Western setting which had premiered on the HBO channel in 2004. For his Doctor Who story, Whithouse sought to balance the classic tropes of the genre with the moral complexity of modern Westerns.

The Gunslinger was initially a robot, but a cyborg offered greater emotional depth

In the autumn of 2011, Whithouse began writing a script which became known as “The Gunslinger”. The eponymous character was initially a robot, but quickly became a cyborg -- called Kahler-Tec, then modified slightly to Kahler-Tek -- because Whithouse felt that this offered greater emotional depth. His first impulse was to depict both Kahler-Tec and Kahler-Jex as recent arrivals on Earth, but Moffat suggested that it would be more effective if Kahler-Jex had been residing in the fictional town of Mercy for an extended period of time, because it would give the residents a stronger motivation to defend him.

At an early stage, Whithouse considered killing off Kahler-Tec partway through the story, which would then focus on Kahler-Jex's villainy. This approach was soon inverted, with Kahler-Jex shot to death by Isaac's crazed father George, a character who would ultimately be dropped from the narrative. The drama of the final act was then generated by the cyborg's intention to punish the town that had deprived him of his revenge. This version of “The Gunslinger” culminated in the Doctor confronting Kahler-Tec with the projected image of a woman named Kahler-San, who wore the same pendant as the cyborg. Although Kahler-Tec saw through the deception, it was enough to convince him to abandon his assault on the people of Mercy.

While he was happy with the beginning of “The Gunslinger”, Moffat felt that Whithouse's script started to run out of steam following the death of Kahler-Jex. It was agreed that preserving both Kahler-Jex and Kahler-Tek through to the climax would give Whithouse more space to explore the moral grey areas of his scenario. Moffat was also eager to embrace the notion of the Doctor becoming a less sympathetic figure when he was deprived of regular travelling companions; as such, it would be the Time Lord, rather than Rory, who would argue for Kahler-Jex to be turned over to the Gunslinger. Furthermore, this prompted the Doctor's uncomfortable assumption of the marshal's duties. For this element, Whithouse drew upon Harper Lee's 1960 novel To Kill A Mockingbird and its 1962 film adaptation. In particular, the Doctor's confrontation with the townsfolk outside the jail was inspired by a scene in which upright Southern lawyer Atticus Finch confronted a lynch mob which had come to kill his black client.

Ben Browder (Isaac) was the star of the science-fiction series Farscape and Stargate SG-1

“The Gunslinger” was paired with Dinosaurs On A Spaceship to form the first production block for Season Thirty-Three, under director Saul Metzstein. To play Isaac, he secured the services of American actor Ben Browder, best known as the star of two recent science-fiction series: the cult favourite Farscape, and the latter seasons of Stargate SG-1. Although born in Tennessee and raised in North Carolina, Browder had attended the Central School of Speech and Drama in London, and was married to British actress Francesca Buller. As such, he was well aware of Doctor Who, and was thrilled to have the chance to appear in the show.

Shortly before the start of recording in 2012, the story's title became “Mercy”. This better reflected the tone of Whithouse's script, while avoiding comparisons with the 1982 novel The Gunslinger, the first volume in Stephen King's popular Dark Tower series. Then, on March 6th, the majority of the cast and crew required for the Spanish location shoot flew out of Heathrow to Madrid, and from there proceeded south to Almería.

Filming for “Mercy” began on March 7th, which was the only day spent at Oasys. Its Fort Apache area provided the exterior of the marshal's office, while scenes inside Kahler-Jex's spaceship were also recorded. Metzstein's team shifted to Fort Bravo on March 8th; work there continued until the 14th, with the exception of a day off on Sunday the 11th. Each day was chiefly preoccupied with material along the rest of Mercy's main street and in the surrounding wilderness. Other venues represented at Fort Bravo included the saloon on the 9th and 12th, and the church on the 14th. The final day in Spain was March 15th, when additional shots in Mercy's outskirts were completed at a ravine between Oasys and Fort Bravo. This left only the sequences in the marshal's office, which were recorded back at Doctor Who's usual studio home in Upper Boat between March 19th and 21st.

In editing, the story's title became the more expansive A Town Called Mercy, echoing Western films such as A Man Called Horse and A Man Called Sledge, both released in 1970. Otherwise, the major change made to the episode was the elimination of a hauntingly beautiful melody which accompanied the appearance of Kahler-Tek. Kahler-Jex would have revealed that he had played this music during the torturous operation which created a cyborg in order to drown out the subject's screams, and it had now been co-opted by Kahler-Tek. As with the other four episodes which formed the first part of Season Thirty-Three, A Town Called Mercy was given a uniquely stylised version of the Doctor Who logo; it appeared to be made of wood, and was shot through with bullet holes.

During 2011, efforts had been made to broaden the footprint of Doctor Who by trailing select episodes with specially-filmed prequels, released online a week or so before the corresponding story was broadcast. For 2012, it was decided to experiment with alternative approaches to this kind of additional content. On August 3rd, Neill Gorton of Millennium FX directed a prequel for A Town Called Mercy at Roath Lock Studios, Doctor Who's new facilities in Cardiff. Entitled The Making Of The Gunslinger, the piece ran one hundred and six seconds, and depicted the transformation of Kahler-Tek into a cyborg under the supervision of Kahler-Jex. However, rather than being released for free online, the BBC instead partnered with Apple's iTunes service, which would offer the prequel for sale.

A Town Called Mercy was broadcast on September 15th. Doctor Who's regular lead-in, Total Wipeout, was moved earlier in the Saturday schedule to accommodate the tenth-season premiere of Strictly Come Dancing. The next day, The Making Of The Gunslinger became available on iTunes. This approach followed the pattern already established two weeks earlier by the prequel for the season premiere, Asylum Of The Daleks.

• Doctor Who Magazine Special Edition #33, Spring 2013, “A Town Called Mercy” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #71, 2015, “Story 228: A Town Called Mercy”, edited by John Ainsworth, Hachette Partworks Ltd.