Parts: 1 (45-minute)
Writer: Mark Gatiss
Airdate: 04 May 2013
Production Code: n/a

Doctor: eleventh
Companions: Clara

Special Guests: the Paternoster Gang

Story: In 1893, Madame Vastra is approached by a man whose brother died under mysterious circumstances. She is intrigued by two facts in particular: first, that the dead man's skin was a vivid red color — the latest victim of an affliction being touted as "the Crimson Horror" — and second, that the last thing he saw in life was, apparently, the Doctor.

Determined to solve the mystery of the Crimson Horror and uncover the Doctor's involvement in it, Vastra travels to Yorkshire with Jenny and Strax, to the place that the dead man was investigating: Sweetville, a community established by moral crusader Mrs Gillyflower, as a refuge for those who wish to spurn the moral decay of society and avoid the inevitable apocalypse. As Mrs Gillyflower recruits only the best specimens of humanity, Jenny is elected to go undercover, and she is soon able to infiltrate the off-limits area of Sweetville... where she discovers that the Doctor has already succumbed to the effects of the Crimson Horror himself.