Doctor: Matt Smith (Eleventh Doctor)
Companion: Jenna-Louise Coleman (Clara Oswald)
Diana Rigg – Mrs Gillyflower
Rachael Stirling – Ada
Catrin Stewart – Jenny Flint
Neve McIntosh – Madame Vastra
Dan Starkey – Strax
Eve de Leon Allen – Angie
Kassius Carey Johnson – Artie
Brendan Patricks – Edmund & Mr Thursday
Graham Turner – Amos
Scott Stevenson - Dipped Man
Olivia Vinall – Effie
Michelle Tate – Abigail
Jack Oliver Hudson – Thomas Thomas
Directed by Saul Metzstein
Written by Mark Gatiss
Produced by Marcus Wilson
Incidental music composer Murray Gold
Series Series 7
Length 45 minutes
Originally broadcast 4 May 2013
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 river with bright red skin. Superstition states that the retina retains the image last seen by the person (an "optogram"), and they are shocked to find that not only did this really happen, but that the latest victim had seen the Doctor. They travel to Yorkshire, where Jenny infiltrates Sweetville, a utopian community led by Mrs Gillyflower and the never-seen Mr Sweet. Mrs Gillyflower preaches about the coming apocalypse to encourage people to come, using as an example of their doomed and corrupt society her blind daughter Ada, who had been beaten by her late husband.
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 had arrived in Yorkshire and were also investigating the mystery of "The Crimson Horror". They had also joined Sweetville to investigate, but learned that they were to be preserved to survive the apocalypse. The process did not work on the Doctor because he was not human, and he was saved from being destroyed by Ada, who affectionately called him "my monster" and had hidden him from her mother. The Doctor finds the preserved Clara in one of Sweetville's houses and manages to reverse the process on her. Meanwhile, Vastra recognises that Sweetville is using the venom of a prehistoric red leech that was a major pest to her people. 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 those "perfect" people she had preserved, 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. Ada, overhearing this, angrily advances towards her mother, giving Clara time to smash the controls. 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 reveal themselves with the vat of leech poison that they have removed from it, rendering it worthless. Mrs Gillyflower turns on the Doctor, but Strax appears at the top of the chimney and shoots at her, causing her to tumble over the staircase. As Gillyflower dies from her injuries, proud of her daughter's hatred towards her, Mr Sweet abandons its host before being smashed to death by Ada's cane. The Doctor and Clara say goodbye; Ada says that she is looking forward to finding new opportunities in life. Vastra and Jenny ask about Clara, as they had previously met a Victorian version of her in "The Snowmen", in which she died. The Doctor does not wish to explain, however (as he hasn't yet worked it out himself).
The Doctor drops off Clara in modern-day London. When she returns home, she finds the two children that she babysits for, Angie and Artie, have discovered photos of her on the Internet from the past, including one that she does not recognise of herself in Victorian London ("The Snowmen"). They assert she must be a time traveller and threaten to tell their father if she does not take them on a trip.