The Perfect Couple

The year is 1893 and Silurian Madame Vastra, her human partner Jenny Flint and their Sontaran butler Strax investigate "The Crimson Horror" - a mysterious cause of death in which victims are found with red skin. Because superstition states that the retina retains the image last seen by the person they are shocked to find that the latest victim, just before he died, had seen The Doctor.

They travel to Yorkshire where Jenny goes undercover, as a convert, to investigate Sweetville, an idyllic community led by Mrs Gillyflower and her never-seen ‘silent partner’ Mr Sweet. Jenny witnesses Mrs Gillyflower preaching about the coming apocalypse to encourage the chosen few to come to Sweetville to help them survive. Mrs Gillyflower even uses her blind daughter Ada, who it seems had been beaten by her late husband, as an example of their doomed society.

Jenny manages to infiltrate a sealed area of Sweetville and discovers The Doctor, chained up in a cell, and exhibiting red skin and a stiff stature. She deduces that he has undergone some form of process that has badly affected him. She also realises that The Doctor wants her to guide him somewhere.

Mrs Gillyflower

Helped by Jenny, The Doctor enters a chamber to reverse the what has been done to The Doctor. On exiting, and apparently back to normal, The Doctor explains to Jenny that he and Clara Oswald had arrived in Sweetville and discovered the mystery of "The Crimson Horror".

They had met with Mrs Gillyflower to join the Sweetville community so as to investigate what has been happening there. But they soon find themselves preserved, by being dunked into a large vat of red liquid. The process however, did not work on The Doctor, because he is not human. Mrs Gillyflower however disposes of such ‘rejects’. But Ada saves The Doctor, who she affectionately calls ‘My Monster’, and has been looking after him as she has become infatuated with him.

A Very Nasty Death

After being rescued by Jenny, The Doctor and Jenny, go off in search of Clara, who has also been preserved (this though confuses Jenny as she last saw Clara killed by the ice maiden months earlier (see "The Snowmen"). The preservation process on Clara is also successfully reversed. They are then joined by Madame Vastra and Srax after Madame Vastra recognises that the substance, the people running Sweetville are using to create "The Crimson Horror", is the venom of a prehistoric parasite that the Silurians considered a major threat 65 million years ago.

On learning this The Doctor and Clara confront Mrs. Gillyflower, who explains her plan and reveals that Mr Sweet is in fact a red leech, who has attached himself to her chest so forming a symbiotic relationship with Mrs Gillyflower. Their plan is to launch a rocket into the skies over England so as to spread the leech's poison over much of the planet.

The Doctor berates Mrs Gillyflower for experimenting on Ada to get the preservation formula right. Ada, overhearing this, angrily advances toward her mother giving Clara time to smash the rocket's controls. However, Mrs Gillyflower holds a gun to Ada's head and retreats into the rocket silo, which has been disguised as a chimney, to activate a secondary launch control.

Jenny

Mrs Gillyflower launches the rocket, but learns moments later Madame Vastra and Jenny reveal themselves with a vat of poison. Enraged, that they have ruined her plans by removing the poison payload, Mrs Gillyflower tries to shoot The Doctor - narrowly missing Ada. Strax, having climbed the chimney from the outside, returns fire, causing Mrs Gillyflower to tumble over the staircase and fall to the bottom of the silo. As Mrs Gillyflower dies, proud of her daughter's hatred towards her, Mr Sweet abandons Mrs Gillyflower. Ada then shares final words with her mother, saying that she'll never forgive her mother, before killing the parasite with her cane.

The Doctor and Clara say goodbye; Ada saying that she is looking forward to finding new opportunities in life. Madame 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 however refuses to explain.

The Doctor then drops Clara off in modern-day London and when she enters her home she finds that Angie and Artie, the two children she helps care for, have discovered photos, on the Internet, of her and The Doctor in different points of time. These include when she was aboard a Soviet nuclear submarine in 1983 ("Cold War") and Caliburn House - the manor house, that she visited with The Doctor, in 1974 ("Hide").

There is also a photo, that Clara does not recognise, that is apparently of herself in Victorian London. Angie and Artie assert that Clara must be a time traveller and they threaten to tell their father if she does not take them on a trip in the time machine.