In 1893, the Silurian Madame Vastra (Neve McIntosh), her human wife Jenny Flint (Catrin Stewart), and their Sontaran butler Strax (Dan Starkey) are hired to investigate the Crimson Horror. Victims have been found dumped in the canal with bright red skin. The most recent victim, Edmund (Brendan Patrick), is found to have the image of the Doctor (Matt Smith) in his retina—an optigram. The investigators travel to Yorkshire, where Jenny infiltrates a utopian community called Sweetville, led by the famous chemist Mrs Gillyflower (Dame Diana Rigg) and a certain Mr Sweet whom no-one has seen.

Jenny finds the Doctor (Matt Smith) there, chained up, his body rigid and his skin red. She puts him in a chamber that reverses the process, and he explains that he and his companion Clara (Jenna-Louise Coleman) were also investigating the Crimson Horror. Infiltrating Sweetville, they learned that they were to be preserved to survive an apocalypse, but the process did not work on the Doctor. Mrs Gillyflower’s blind daughter Ada (Rachael Stirling) had hidden him.

The Doctor finds Clara and reverses the process for her too. Vastra, meanwhile, has learned that Sweetville is using the venom of a prehistoric red leech known to her species. The Doctor and Vastra confront Mrs Gillyflower, who plans to use a rocket to spread the poison through the skies, and everyone will die except those in Sweetville, who will repopulate the Earth. Mr Sweet, it seems, is a red leech from Silurian times who has a symbiotic relationship with Mrs Gillyflower.

The Doctor berates her for experimenting on her own daughter, while Clara smashes the controls of the rocket, but Mrs Gillyflower holds a gun to Ada’s head and darts into the rocket silo, where there is a secondary control. She launches the rocket, but Vastra and Jenny have already removed the red leech poison. Strax shoots her and she stumbles backward, falling to her death, while Ada kills Mr Sweet with her cane. Clara returns home to find that Angie (Eve de Leon Allen) and Artie (Kasius Carey Johnson), whom she babysits, have found photos of her travelling in the TARDIS and blackmail her into taking them on a trip.

Diana Rigg (Mrs Gillyflower) and Rachael Stirling (Ada) are actually mother and daughter. The casting was a favour to them, who had never worked together. For the first time, Diana Rigg got to speak in her native Doncaster accent. Little references to Diana Rigg’s work in the British Avengers series were included in the script, including Jenny fighting in a leather catsuit. Rigg, of course, has appeared in countless other movies and TV shows, notably as Tracy, James Bond’s wife in On Her Majesty’s Secret Service, the Evil Queen in 1987’s Snow White, and Olenna Tyrell in Game of Thrones. Reviews were generally positive. The episode was called brilliant, the humour and style were praised, and one reviewer said it had more dash than the Avengers.

This was the 100th episode of the 2005 series reboot. Sweetville is based on the Saltaire, Yorkshire, model village, founded in 1851 by Titus Salt. He had a daughter named Ada. Matt Smith based the Doctor’s relationship with Strax on Blackadder’s with Baldrick. Diana Rigg had never seen Doctor Who, so they showed her clips of classic Who villains. At the start of The Adventure of the Pince-nez (1904), Sherlock Holmes mentions the repulsive story of the Red Leech. The name of Mrs Gillyflower may come from the actress Gilly Flower in Fawlty Towers.