In 1893 Yorkshire, Mrs Winifred Gillyflower warns that humanity's moral depredations are sure to bring about an imminent doomsday. She and her disfigured daughter, Ada, recruit followers for a community they have established called Sweetville. But something strange is afoot: no one who moves to Sweetville ever returns, and corpses have been found floating downriver, their skin turned a lurid red. With the body count rising, Madame Vastra, Jenny and Strax are asked to investigate Mrs Gillyflower's activities. But they soon discover that the latest victim of the so-called “Crimson Horror” is none other than the Doctor himself.

One of the most popular elements of Doctor Who's thirty-second season was the introduction of Madame Vastra, her associate Jenny, and Commander Strax. These previously-unknown allies of the Doctor were created by executive producer Steven Moffat for A Good Man Goes To War, and were so well-received that consideration was briefly given to spinning them off into their own series. Instead, Moffat decided to feature the trio -- now dubbed the Paternoster Gang, in reference to Vastra's residence on Paternoster Row in the City of London -- on a recurring basis during Season Thirty-Three. They would appear in the 2012 Christmas special, The Snowmen, and then return for an adventure during the latter part of the season, which would air in the spring of 2013.

Moffat initially hoped to write both scripts himself, but his slow progress on The Snowmen made this impossible. Instead, he turned to Mark Gatiss, who was then finishing Cold War, the third of Doctor Who's 2013 episodes. Gatiss was an avowed fan of Victoriana; indeed, he had set his first televised story -- 2005's The Unquiet Dead -- during the same period. He was amenable to Moffat's idea that the new adventure should offer a change of pace by unfolding from the perspective of the Paternoster Gang.

Mark Gatiss thought that Sir Arthur Conan Doyle's involvement might be tied to his primary career as a physician

Gatiss' first impulse was to write a “celebrity historical”, which would spotlight a well-known personality of the era in a manner similar to Charles Dickens in The Unquiet Dead. At the time, he and Moffat were also showrunning Sherlock, a modern reinvention of Sir Arthur Conan Doyle's Sherlock Holmes mysteries. This suggested Doyle's involvement in the narrative, which Gatiss thought might be tied to his primary career as a physician, and specifically his specialisation in ophthalmology. The starting point therefore became the debunked forensic technique of optography, popular in Victorian times, which asserted that the human retina retained an impression of the last image seen before death.

However, with so many characters fighting for space in his narrative, Gatiss quickly concluded that the celebrity historical was the wrong approach to take, since there was little for Doyle to do once the scenario was introduced. Instead, he decided to place the Doctor, Clara and the Paternoster Gang in an English setting which Doctor Who had rarely explored: the northern country around his native County Durham, and Yorkshire in particular. Gatiss discussed the notion with his friend, fellow Doctor Who fan Matthew Sweet, who was also a writer and broadcaster. Sweet's first published book had been 2001's Inventing The Victorians, a deconstruction of the popular view of the Victorian era. He prompted Gatiss to think in terms of the era's factory workers, and the miserable conditions in which they toiled.

Other elements of the adventure came together thanks to a quirk of circumstance. Gatiss felt that Doctor Who boasted too few strong female antagonists, and he thought that one would be particularly effective in opposition to Madame Vastra. At the time, he was appearing in a stage production of The Recruiting Officer with actress Rachael Stirling, who was perhaps best known for the Victorian drama Tipping The Velvet. Gatiss was also a friend of Stirling's mother, Dame Diana Rigg, who had become a Sixties icon by virtue of her role as stylish spy Emma Peel in two seasons of The Avengers, as well as her pairing with George Lazenby's James Bond in the 1969 film On Her Majesty's Secret Service. One evening, Gatiss had dinner with Stirling and Rigg. Aware that the pair had never acted together, he mooted the idea of including characters in his Doctor Who script written especially for them. Stirling was enthusiastic, while Rigg -- who had never seen Doctor Who, and rarely appeared in episodic television -- was persuaded after viewing a showreel of the programme's iconic villains which had been assembled by The Recruiting Officer's stage manager, Michael Dennis. Rigg was also keen on the Yorkshire setting, having been born in Doncaster.

Jenny's leather catsuit was a reference to a memorable outfit worn by Diana Rigg in The Avengers

Early on, Gatiss' storyline was referred to as “Mother's Ruin”, but it had become The Crimson Horror by the time he completed the first draft of his script at the end of March. Sweetville was inspired by real planned communities in the North, such as Saltaire (built in 1851 by Sir Titus Salt to house workers at his textile mills) and Akroydon (developed from 1859 by Colonel Edward Akroyd to service his mills). The unnatural slurry which caused the eponymous “horror” was suggested by the 1966 comedy film Carry On Screaming!, in which a chemical sludge turned young women into mannequins. Mr Sweet -- named in homage to Matthew Sweet -- was derived from “the repulsive story of the red leech”, an untold exploit of Sherlock Holmes mentioned at the start of Doyle's 1904 short story The Adventure Of The Golden Pince-Nez. The Thursday brothers took their surname from GK Chesterton's surreal 1908 thriller The Man Who Was Thursday: A Nightmare. Mrs Gillyflower's protest that she could not bear the sight of sick people was adapted from an incident relayed to Gatiss by Stirling, in which Rigg had been discomfited by a hospital stay following a knee operation. Meanwhile, Jenny's leather catsuit was a reference to one of the most memorable outfits worn by Rigg during her stint in The Avengers.

In Gatiss' original conception, Mrs Gillyflower escaped after the launch of her rocket, leading to a carriage chase which ended when the villainess was thrown and impaled on a statue. As a token of his thanks, the Doctor gave Vastra a portable perception filter which she could use to disguise her Silurian nature. In the first draft, Clara was revived only at the very end of the story but, when it was decided that she should have a greater presence in the narrative, Gatiss' second version saw her take over much of Jenny's role. Vastra's partner now went missing early in the episode while infiltrating Sweetville, and it was Clara who donned the catsuit and fought the Pilgrims. Later drafts found a compromise, with Jenny reinstated throughout the script, and Clara's resuscitation positioned earlier than in Gatiss' original plans.

The Snowmen was originally intended to enter production several weeks before The Crimson Horror, but was delayed as Moffat struggled with its script. In the end, the timing was appropriate to pair the two stories as the season's sixth recording block under director Saul Metzstein, given the commonalities of cast and setting. At this point, Gatiss' adventure was intended to be the fourth of the eight episodes which would comprise Doctor Who's Spring 2013 run; it was later pushed back to fifth and finally sixth. As it transpired, the bulk of The Crimson Horror was recorded before The Snowmen, and hence marked the first time that Neve McIntosh, Catrin Stewart and Dan Starkey had worked on Doctor Who since the completion of A Good Man Goes To War in late January 2011. Matt Smith was delighted to see the trio return, and he based the Doctor's relationship with Strax on the interplay between Rowan Atkinson's Blackadder and Tony Robinson's Baldrick in the historical sitcom The Black Adder and its sequels.

Scenes on the streets of Sweetville were the first order of business for Saul Metzstein's team

Scenes on the streets of Sweetville were the first order of business for Metzstein's team, taking them to Rhymney on July 2nd. The 3rd saw filming at two Barry locations: Mrs Gillyflower gave her address from the pulpit at Holy Trinity Church while, at the offices of Barry Dock, Jenny entered the panelled corridor and Mrs Gillyflower descended the spiral staircase. July 4th and 5th brought cast and crew to Doctor Who's production home of Roath Lock Studios. Scenes on the top floor of the mill were taped on both days, alongside those in the cottage and Vastra's dark room on the first day, and within the shuttered room on the second day. Various Yorkshire exteriors were captured at Llantrisant on July 6th. It was back to Roath Lock on the 7th for some of the climactic confrontation on the staircase leading to the rocket.

Recording for The Crimson Horror then paused for two weeks to facilitate various promotional engagements, including a major Doctor Who panel at the San Diego Comic-Con. As such, it wasn't until July 25th that filming resumed at Roath Lock, for more sequences beneath the rocket and the start of work in Mrs Gillyflower's parlour. On the 26th, a gymnasium at Tonyrefail School in Tonyrefail was used for material in the morgue, the laboratory, and the office where the Doctor and Clara passed selection. Then it was back to Roath Lock for more parlour scenes on July 27th and 28th, with the sets for the vat room, the rocket staircase, and the upper window also in use on the latter day.

After the weekend, work at Roath Lock continued from July 30th to August 2nd. On the first day, Metzstein recorded footage on the empty mill floor plus various inserts, before starting on material in the top floor corridor. He concluded the latter on the second day, after which his itinerary included sequences on the spiral staircase and the shuttered room. The next set in use was for the revivification area, where the cameras continued rolling on the third day. Scenes in the mill corridor were completed on the last day, alongside more pick-up shots; footage of the body being recovered from the canal was also captured near Brigantine Place in Cardiff.

Metzstein then focussed on The Snowmen for the next three weeks. It wasn't until August 22nd that work on The Crimson Horror resumed at Roath Lock, with more filming on the sets for the revivification area and the vat room. On August 24th, Madame Vastra's orchid house was really Llandough Castle in Llandough, while Strax proposed a frontal assault on Sweetville at Treowen Manor in Dingestow on the 27th. The chapel exterior was shot at Roath Lock on August 29th. A number of inserts were filmed in the studio on October 18th and 25th; the latter day also saw Strax ascending the chimney at Barry Pumping Station in Barry.

This should have concluded production on The Crimson Horror, but it soon became clear that the episode was underrunning. Since McIntosh, Stewart and Starkey would be returning for the Season Thirty-Three finale, The Name Of The Doctor, two new sequences were developed for The Crimson Horror which featured Vastra, Jenny and Strax. These were the scenes in Vastra's carriage of the Paternoster Gang plotting en route to Yorkshire, plus Vastra and Strax discussing how Jenny would locate the Doctor. Furthermore, since The Crimson Horror had now been fixed as the sixth episode of the spring schedule, it acquired an epilogue originally intended for the story it displaced, Journey To The Centre Of The TARDIS, which would now precede it. This coda involved Clara's charges, Angie and Artie Maitland, and was designed to lead into episode seven, Nightmare In Silver.

The result was two additional days of filming. The new 1893 material was recorded on November 16th at Roath Lock, with shots of the carriage also filmed at Mount Stuart Square in Cardiff Bay. The closing scene in the Maitland household was taped on November 26th, at a residence on Beatty Avenue in Cardiff.

• Doctor Who Magazine Special Edition #37, Autumn 2014, “The Crimson Horror” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #74, 2018, “Story 237: The Crimson Horror”, edited by John Ainsworth, Hachette Partworks Ltd.