The Crimson Horror

In 1893, Silurian Madame Vastra, her human wife Jenny Flint, and their Sontaran butler Strax are asked to investigate the "Crimson Horror", a mysterious cause of death in which victims are found dumped in the canal with bright red skin. The latest victim, Edmund, has the image of the Eleventh Doctor retained in his retina. Vastra, Jenny, and Strax travel to Yorkshire, where Jenny infiltrates Sweetville, a utopian community led by chemist and engineer Mrs Gillyflower and the never-seen Mr Sweet.

Mrs Gillyflower's costume, Mr. Sweet and the controls to launch the rocket, as shown at the Doctor Who Experience.

Jenny discovers the Doctor, who is chained up and exhibits red skin and a stiff stature. At his silent direction she puts him into a chamber to reverse the process. Once restored, he explains to Jenny that he and Clara were also investigating the Crimson Horror. They had also infiltrated Sweetville, but learned that they were to be preserved to survive an apocalypse. The process did not work on the Doctor because he was not human, and he was saved from being destroyed when Mrs Gillyflower's blind daughter Ada had hidden him. The Doctor finds Clara and reverses the process on her. Meanwhile, Vastra recognises that Sweetville is using the venom of a prehistoric red leech her people knew. The Doctor and Clara confront Mrs Gillyflower, who reveals that she plans to launch a rocket to spread the poison all over the skies; everyone on Earth will die except everyone who lives in Sweetville, who will then start over to make a better world. "Mr Sweet" is also revealed to be a red leech from Vastra's prehistoric times that has formed a symbiotic relationship with Mrs Gillyflower. The Doctor berates Mrs Gillyflower for experimenting on Ada to get the preservation formula right. Clara smashes the controls to the rocket. However, Mrs Gillyflower holds a gun to Ada's head and heads into the rocket silo, which has been disguised as a chimney, to reach the secondary control.

Mrs Gillyflower launches the rocket, but Vastra and Jenny have already removed the red leech poison from it, rendering it worthless. Strax shoots at Mrs Gillyflower to stop her from killing the others, causing her to stumble backwards off the stairs, through the banister, and die. Ada kills Mr Sweet with her cane, causing gross green goo to spurt out. When Clara returns home, she finds the two children that she babysits for, Angie and Artie, have discovered photos of Clara from the past on the Internet. They force her to take them on a trip.

When the Doctor arrives in Yorkshire, he mentions to Clara that he once spent ages trying to get a "gobby Australian" to Heathrow Airport, a reference to the Fifth Doctor's companion Tegan Jovanka and his efforts to get her back to Heathrow from Four to Doomsday to Time-Flight (1982).[3] Further reference to Tegan is made when the Doctor tells Clara, "Brave heart, Clara," a phrase he often used when talking to Tegan.[3]

In the flashback sequence, the Doctor says that the Romani people believe that the last image a dead person sees is retained on the retina. This is similar to a version that the Fourth Doctor tells the crew of Nerva Beacon just before he connects his mind to the retina of the dead Wirrn in The Ark in Space (1975).[3]

Upon arriving home, Clara discovers that the children she looks after have found historical photographs of her from 1974 ("Hide") and 1983 ("Cold War"). They also found an 1892 photo of Clara Oswin Oswald / Miss Montague ("The Snowmen"), whom they assume to be their Clara. Clara doesn't recognise this one at all.

"The Crimson Horror" received generally positive reviews from critics. Ben Lawrence of The Daily Telegraph gave the episode five out of five stars, writing that it "crammed in idea after idea while still maintaining a terrific, breezy pace and delivering a fantastically satisfying story". He praised the way the Doctor and Clara did not enter the episode for fifteen minutes, which cut down on the amount of exposition.[12] The Guardian reviewer Dan Martin was positive towards the way the episode played with genre and form, saying that it "was as demented and creepy as the show should always be".[13] Neela Debnath, writing for The Independent, said that the episode had a "great plot to match the gigantic scale".[14]

Patrick Mulkern of Radio Times wrote that it had "decent mystery, a logical plot, a dollop of camp but perhaps most rewarding of all it's a danse macabre". He noted that the episode had "more than a dash of The Avengers", in which Rigg is famous for starring.[15] IGN's Mark Snow gave "The Crimson Horror" a rating of 8.7 out of 10, calling it "the best yet" of this half of the season. He praised the humour and style, and commented, "the threat was never truly looming, nor was the scale as grand or epic as its recent predecessors, but that also meant that for once there was just enough story to fit into one sole episode".[16] SFX reviewer Nick Setchfield gave the episode four out of five stars, describing it as a "sufficiently sure-footed to waltz right to the brink of parody and no further". While he praised Rigg, he said that Stirling had "the stand-out performance".[17] Digital Spy's Morgan Jeffery was more critical, giving the episode two out of five stars. He remarked that it felt like "filler" and criticised Rigg's character for being scripted as "an over-the-top cackling crone". However, he praised the emotional depth added by Stirling's character and the direction.[18]

Graham Kibble-White, reviewer in Doctor Who Magazine, also gave it a positive review, calling it "bloody brilliant" and "a remarkably well-told tall tale, in which every element - including its name - radiates a real luminosity." He described the story as "a thoroughly wicked yarn full of sly jokes," and complimented the way that the Doctor wasn't in during the first third, calling it a "bold move." Additionally, he praised Vastra, Jenny, and Strax, saying "once again they have the wherewithal and the panache to hold their own against the mighty magnetic pull of When's the Doctor coming on?" He claimed that, to him, Strax's "continual championing of things like triple-blast brain-splitters is starting to grate." He complained that he already hated Artie and Angie, and that the scene in which they appear felt "as if we've careened from a Sunday afternoon serial into a Saturday morning cartoon."[19]

A novelisation of this story written by Mark Gatiss will be released in paperback and digital formats 11 March 2021 as part of the Target Collection.[20][21]