Struggling to find a suitable place to live, Bill and several friends are approached by an elderly man who offers to rent them rooms in a decaying mansion. His only rule is that the tower at the heart of the structure is strictly out of bounds. On their first night in the house, however, it becomes clear that something strange is afoot. The wooden floors and panelling creak unnaturally, the trees sway without a breeze, mysterious insects roam the corridors... and, one by one, the roommates start to vanish. Fortunately, the Doctor has come along to help Bill unpack -- and together, they uncover a mystery which stretches back decades.

Mike Bartlett had been a Doctor Who fan during his childhood, and had aspired to write for its twenty-first-century revival from an early stage. However, it wasn't until 2015 that he had firmly established his writing pedigree, having won several Laurence Olivier Awards for his plays, and launched the successful drama Doctor Foster on television. In a September 17th interview, Bartlett made mention of his interest in Doctor Who, and the comment caught the eye of script executive Lindsay Alford whose husband, Brian Minchin, was its executive producer alongside Steven Moffat. Bartlett was quickly invited to pitch for Doctor Who, and though Moffat assumed that he would be unavailable after Doctor Foster was renewed for a second season, he was delighted to learn that Bartlett was nonetheless committed to providing him with a script.

In considering ideas for Doctor Who, Bartlett knew that an often successful approach, frequently employed by Moffat himself, was to present an everyday phenomenon in a sinister light. His thoughts eventually turned to the way older houses -- such as the dilapidated Victorian buildings in which he had lived during his university days -- were perpetually filled with creaking noises. Since the United Kingdom was experiencing a shortage of living accommodations, he imagined an adventure involving a group of people who were brought together by an opportunity to share a spooky old house. This dovetailed nicely with the St Luke's University setting being introduced for Doctor Who's thirty-sixth season, since it meant that the flatmates could include Bill and a cohort of students.

The Landlord was drawn from Mike Bartlett's memories of a man in a suit who had once visited his parents

By March 2016, Bartlett's story was called Knock Knock; “The Haunted Hub” would later be erroneously cited as a working title. The wooden Eliza was inspired by The Sandalwood Girl, the second volume of Sheila K McCullagh's Puddle Lane series of books for young readers, first published in 1985. The Landlord was drawn from Bartlett's memories of a man in a suit who had once visited his parents. The historical element of the mystery was suggested by the American paranormal programme The X-Files, in which the lead characters often investigated strange events which had been occurring for many years.

Initial drafts of the script featured an elderly widow called Mrs Lumb, who was acting as Bill's guardian. She was later replaced with Moira, whom Moffat had developed for the season premiere, The Pilot. At this stage, Eliza was indeed the Landlord's daughter, having fallen ill at the age of five. The Landlord's lifespan was being extended by the Dryads, who kept him in a partially human state so that he could recruit new victims when required. After Eliza decided to end her life, all of the people who had been consumed by the Dryads over the years emerged from the walls; those who hailed from earlier decades were returned home by the Doctor.

Originally, Harry was revealed to be the grandson of Harry Sullivan, who had accompanied Tom Baker's Fourth Doctor during 1975. Portrayed by the late Ian Marter, the character was a particular favourite of Bartlett's, but concern was expressed that he was too obscure for a general audience to appreciate the reference. It was then thought that Harry would instead be an ancestor of one of the characters in the season's second episode, Smile; it would be implied that his experience with the Dryads had inspired an interest in nanobot technology, of the sort which would feature prominently in the earlier episode. This notion, too, was eventually abandoned.

Knock Knock was always intended to be made as part of the season's second recording block alongside episode three, Thin Ice. At an early stage, however, it was planned to be broadcast sixth; only around mid-June was it moved up the running order to become the year's fourth installment. An additional consideration for director Bill Anderson would be the use of binaural sound recording technology. Minchin had been keen to experiment with the format, which attempted to evoke a three-dimensional acoustical experience for playback via headphones, and he felt that Bartlett's use of sound to build a spooky atmosphere made Knock Knock a suitable candidate. Binaural sound would not be available during the episode's broadcast, but would be an option for viewers streaming the story via the BBC's iPlayer service.

With Anderson initially focussing on Thin Ice, the only filming for Knock Knock during the early part of Block Two occurred on August 3rd. It saw the sequences in the TARDIS and in Bill's room recorded at Roath Lock Studios in Cardiff. Here Jennifer Hennessy reprised the role of Moira she had originated in The Pilot, but her material -- in which she advised Bill that she had already rented out her bedroom -- was dropped in editing.

Two locations were used to represent most areas of the house at 11 Cardinal Road. The first was Fields House in Newport, where Anderson recorded scenes on the grounds of the estate, as well as in the drawing room, the cellar and the Landlord's room. Cast and crew were present there on August 11th and 12th, and again after the weekend on the 15th and 16th. The next several days -- August 17th to 19th and, following another pause for the weekend, the 22nd and 23rd -- were spent at Cefn Tilla Court in Llandenny. It offered the house's stairs, various landings, the kitchen, the hall, Bill's room and Pavel's room. In this footage, access to the tower was provided by a sliding bookcase, rather than a secret door hidden in a stone wall, as Bartlett had scripted.

A busy August 24th involved filming at several Cardiff locations. First up were the two rental options rejected by Bill and her friends: the house next to the factory was on Percy Street, while the tiny house was on Fairwater Grove West. Next, a residence on Fairwater Grove East was used as the exterior of Moira's home. The estate agency, Redcliffe Lettings, was represented by an actual realtor on Sneyd Street. Finally, Anderson began shooting material in the tower room at Roath Lock; this work continued through to the 26th. Various inserts were then taped in the studio on September 6th.

By early 2017, Moffat had decided that more emphasis should be placed on the mystery of the vault which the Doctor and Nardole were guarding at St Luke's. As a result, he wrote a new closing scene for Knock Knock, which was recorded at Roath Lock on February 22nd. To accommodate it, the story's original ending was dropped. It would have revealed that the old house had been restored after Eliza and the Landlord were consumed by the Dryads. The Doctor had manipulated the title deeds to take possession of the property -- much as he had done in Thin Ice -- which meant that Bill and her flatmates had a place to live after all. They quickly discovered that Eliza still lingered as a helpful if unseen presence within their new home. With this material cut, the mansion would now simply be shown to have been destroyed.

• Doctor Who: The Complete History #86, 2018, “Story 268: Knock Knock”, edited by Mark Wright, Hachette Partworks Ltd.