In the seventeenth century, the Doctor vies with a highwayman called the Knightmare for deadly alien technology masquerading as jewellery. But the hunt is complicated by the fact that the Knightmare is really Ashildr -- now centuries old and going by the name “Lady Me” as the memories of her original life fade with the passage of time. The Doctor must confront the ramifications of his decision to bestow immortality upon Ashildr, not the least of which is an alliance with the alien Leandro which may have devastating consequences for the Earth.

For Doctor Who's thirty-fifth season, executive producer Steven Moffat wanted to explore the nature of the Doctor's longevity. It was well-established that he was many centuries old -- at the very least -- but his ability to travel in time meant that he had a scattershot footprint in the chronology of the universe. Moffat thought it would be interesting to contrast the Doctor with an ancient who was still forced to live life one day at a time. This character would be made immortal through the Doctor's own actions in the season's fifth episode, while the sixth installment would see them encounter each other again after the passage of centuries.

To write episode six, executive producer Brian Minchin encouraged Moffat to approach Catherine Tregenna; she had contributed four stories to the spin-off series Torchwood, on which he had been a script editor. Tregenna had been invited to pitch for Doctor Who before, but she had previously declined because she didn't think her emotive brand of storytelling suited the show. Now, however, she was persuaded by the unique nature of the immortal character, as well as by the opportunity to place the Doctor in a position of personal conflict as he confronted the consequences of his actions.

In order to keep the story's focus firmly on Ashildr, it would omit the Doctor's regular companion, Clara Oswald

Tregenna began developing her narrative around the same time that Jamie Mathieson was working on the script ultimately entitled The Girl Who Died, which would introduce the deathless Ashildr. She was only aware of the basic details of Mathieson's story, but considered this to be sufficient because she felt that, after hundreds of years, the character would have essentially forgotten her origins anyway. In order to keep the story's focus firmly on Ashildr, it was decided that Tregenna's adventure would effectively omit the Doctor's regular companion, Clara Oswald.

No specific setting for Tregenna's story was required, so she eventually settled on the seventeenth century. Inspired by the clever thief Twm Siôn Cati of Welsh folklore, she saw appeal in a narrative involving highwaymen; Moffat had likewise wanted to pursue this subject for some time. Tregenna also drew upon The Wicked Lady, a 1945 film which loosely retold the legend of Lady Katherine Ferrers, an alleged highwayman until her death from a gunshot wound in 1660. The year 1651 was ultimately chosen because it saw the final defeat of the Royalist armies by Oliver Cromwell's forces during the English Civil War, and hence was a particularly lawless time.

Tregenna's first draft was completed in late October 2014 under the title “The Doctor And Me”. At this stage, the Doctor and Ashildr did not break into the Fanshawe residence, but instead cooperated on a second attempt to hijack Lucie's coach. Leandro was the king of the planet Lavi -- later renamed Delta Leonis -- where the space-time portals his people utilised had attracted the attention of the Reapers, monsters first seen in 2005's Father's Day. Ashildr gave the Doctor the surviving immortality-granting device -- not yet defined to be a Mire battlefield medical kit -- and he used it on himself in order to seal the portal.

Although Sam Swift was present in Tregenna's initial storyline, he became a more significant character as the script was refined. His attempted ambush of the Doctor and Ashildr replaced a confrontation with Roundhead soldiers; these characters became the two dimwitted pikemen who wound up rescuing the Doctor. Likewise, Swift took the Doctor's place as the recipient of the immortality device, although Tregenna was careful to leave some ambiguity as to whether he would now live forever. The adventure was renamed The Woman Who Lived, in order to draw an explicit connection with The Girl Who Died. Both stories would form Block Three of the Season Thirty-Five production schedule under director Ed Bazalgette.

Filming for The Woman Who Lived began on March 25th, 2015. Fforest Fawr near Taff's Well provided the venue for both the Doctor's horseback ride to Tyburn and his emergence from the TARDIS. Production on the episode began in earnest the following week, starting on the 30th with the TARDIS sequence -- Jenna Coleman's only work on the story -- at Roath Lock Studios in Cardiff. March 31st took Bazalgette's team to Newport, where Tredegar House posed as the Fanshawe estate. The majority of Ashildr's residence, Hounslow Hall, was actually Llansannor Court in Llansannor. Recording there took place from April 1st to 3rd, after which the Easter weekend led into an extended week-long break.

Cast and crew reassembled on April 13th at St Fagans National Museum of History in Cardiff, where the Abernodwydd Farmhouse served as Ye Swan with Two Necks. The same vicinity was also used for two flashbacks: Ashildr being taken to be drowned as a witch and an episode during the Black Death. The latter was dropped in editing, and would have depicted Ashildr leaving London and passing a man in a plague doctor's outfit, who turned out to be the Doctor. On the 14th and 15th, the gallows at Tyburn were erected on the grounds of St Donat's Castle in Llantwit Major. Another flashback, of Ashildr escaping her coffin, was also shot there on the latter day, alongside an insert of Leandro breathing fire.

Recording for The Woman Who Lived then became more haphazard, as Bazalgette focussed on The Girl Who Died. On April 21st, a small set was erected at Roath Lock to represent the Fanshawes' chimney. Work on the 25th began at Cosmeston Medieval Village near Penarth for the flashback to the Battle of Agincourt. Later that day, material on the Fanshawes' roof was taped at Roath Lock. It was back to Fforest Fawr on April 30th and May 1st, this time for the waylaying of the Fanshawes' carriage and Sam Swift's failed hold-up. On May 5th, scenes in Ashildr's library were shot at Castle Farmhouse, a fifteenth-century edifice in St Georges super Ely. A heated pool on the building's grounds was also used for the flashback of Ashildr in the lake. Lastly, several inserts were captured at Roath Lock on May 6th.

• Doctor Who: The Complete History #82, 2018, “Story 257: The Woman Who Lived”, edited by Mark Wright, Hachette Partworks Ltd.