[3] Set in 1893 Yorkshire, the episode features the Victorian-era detectives Madame Vastra (Neve McIntosh), Jenny Flint (Catrin Stewart), and their alien butler Strax (Dan Starkey).

The first half of the episode focuses on the detectives' search for their missing friend, the alien time traveller the Doctor (Matt Smith).

In the second half, they team up with the Doctor and his companion Clara Oswald (Jenna-Louise Coleman) to prevent a plot by the chemist and engineer Mrs Gillyflower (Diana Rigg), who wishes to start a new world by wiping out all of humanity, apart from a community she deems as being "perfect".

In 1893, Silurian Madame Vastra, her human wife Jenny Flint, and their Sontaran butler Strax are asked to investigate a mysterious cause of death known as the "Crimson Horror", the victims of which are found dumped in the canal with bright red skin.

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 the people living 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.

This is similar to a version that the Fourth Doctor tells the crew of Nerva Beacon just before he connects his mind to the dead Wirrn in The Ark in Space (1975).

Executive producer Steven Moffat told Radio Times that the story would be from their point of view, for the audience "to see them tackle a case of their own, and stumble across the Doctor's path, quite accidentally".

[6] The initial pitch of the episode was "almost a Paternoster gang spinoff", featuring the author of the Sherlock Holmes books, Sir Arthur Conan Doyle.

The antagonist of the episode was originally intended to be "mummies" that were "all in black, sort of like Scottish widows", inspired by sufferers of the Phossy jaw condition.

[13] 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 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".

[15] SFX reviewer Nick Setchfield gave the episode four out of five stars, describing it as "sufficiently sure-footed to waltz right to the brink of parody and no further".

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

[24] When writing the adaptation, Gatiss wanted to keep the pacing of it as close as possible to the episode, adding only one extra scene detailing Jenny's first meeting with The Doctor.