The Doctor, Rose and Jack land in modern-day Cardiff, where they mean to refuel the TARDIS using the time rift they encountered in 1869. They learn that the new mayor is none other than Margaret Blaine: one of the Slitheen who infiltrated 10 Downing Street. Really Blon Fel-Fotch Passameer-Day Slitheen, she plans to construct a nuclear reactor which will tap into the rift, destroying the Earth and providing the energy she needs to escape into space. With Mickey's help, the time travellers set out to stop Blon. But the Doctor is confronted with a painful choice when it's revealed that Blon faces a death sentence on her homeworld.

Of all the stories which comprised Doctor Who's twenty-seventh season, the last to take a clear shape was the eleventh episode. The earliest idea developed for the slot by executive producer Russell T Davies was an historical adventure called “Pompeii”. However, Davies quickly realised that the episode would have to be a budget-conscious exercise, since it would fall between a pair of two-part stories -- ultimately The Empty Child / The Doctor Dances and Bad Wolf / The Parting Of The Ways -- which were both likely to be expensive. “Pompeii” was shelved, eventually reappearing as James Moran's 2008 script The Fires Of Pompeii. Instead, Davies' pitch document of December 8th, 2003 simply indicated that episode eleven would be a small-scale, character-oriented adventure. He dubbed it “The New Team”, due to the preceding episode's addition of Jax -- later Jack Harkness -- to the TARDIS crew.

To write episode eleven, Davies approached his friend Paul Abbott, the creator of series such as Touching Evil and Linda Green. Abbott spent about a month working on an untitled storyline which revealed a more insidious side to the Doctor's involvement in Rose's life. However, Abbott then became busy with his new show, the popular comedy-drama Shameless, and had to step away from Doctor Who. In its place, Davies developed “The Void”, which was principally set aboard the TARDIS. It had provisionally occupied the episode eleven slot by July 2004.

Russell T Davies was deeply impressed by Annette Badland's performance as Margaret Blaine in Aliens Of London / World War Three

The same month, Season Twenty-Seven entered production. Part of the first recording block was Aliens Of London / World War Three, which pitted the Doctor against Davies' monstrous new creations, the Slitheen. He was deeply impressed by the performance which Annette Badland gave as the faux-human Margaret Blaine. He inquired as to whether the actress would be interested in returning to Doctor Who, and Badland readily agreed. Davies decided that episode eleven offered an ideal opportunity to revisit Badland's Slitheen character; the costume and computer rendering already existed, so no significant strain would be placed on the programme's budget. Rather than confining the narrative to the TARDIS, Davies also decided to situate the action in modern-day Cardiff. With the city now Doctor Who's production home, he had been keen for a story to be explicitly set there. Episode three, The Unquiet Dead, also took place in Cardiff -- albeit during the nineteenth century -- and Davies decided to revisit the time rift which had been central to that story's plot.

Since so many cost-saving measures were in place for the episode, Davies concluded that the budget could stretch to accommodate a visually striking climax, which took the form of the Cardiff earthquake. Narratively, he intended his script to tread new ground by examining the moral repercussions of the Doctor's deeds -- particularly in the context of capital punishment, to which Davies was personally opposed. With the supper conversation between the Doctor and Blon Fel-Fotch Passameer-Day Slitheen forming the crux of the narrative, Davies considered calling the story “Dining With Monsters”. By the time his first draft was completed on November 26th, however, it had become known as Boom Town.

Davies was now contemplating a Cardiff-based Doctor Who spin-off, and he began to seed potential elements into the script for episode eleven. The new show would eventually become Torchwood, where the effect of the TARDIS on the area around the Millennium Water Tower in Roald Dahl Plass would become a recurring plot point. Davies also added a reference to the new range of original Ninth Doctor novels being prepared by BBC Books, with Rose's mention of Justicia alluding to the setting of Stephen Cole's The Monsters Inside. It replaced dialogue referring to an unseen adventure on Grajick Major. The recurring phrase “bad wolf” was now well-established in other scripts for the season, and Davies added its Welsh translation to Boom Town as the name of the nuclear reactor: Blaidd Drwg.

It was initially planned that Boom Town would be paired with Bad Wolf / The Parting Of The Ways to form the season's fifth and final production block. For logistical reasons, however, episode eleven eventually comprised Block Five on its own. An early candidate to direct Boom Town was Paul Kelly, a musician and filmmaker who was best known for his work with the band Saint Etienne. Instead, at a late stage, the story was offered to Joe Ahearne. He had already directed Block Three -- Dalek and Father's Day -- and was now preparing to record Bad Wolf / The Parting Of The Ways as Block Six.

Location filming for Boom Town took place entirely in Cardiff. Work got under way on January 19th, 2005, when the Doctor and Blon's dinner was taped in and around Bistro 10; the restaurant was situated at Mermaid Quay in Cardiff Bay, close to where Davies lived. Careful scheduling was necessitated by Badland's limited availability, due to her commitments to the fourth and final season of the drama Cutting It. Both February 1st and 2nd were split between Glamorgan House -- posing as Cardiff City Hall -- during the day and Mermaid Quay, especially Roald Dahl Plass, during the night. The 1st also saw Davies and his fellow executive producer, Julie Gardner, receive confirmation from Jane Tranter, the BBC's Controller of Drama Commissioning, that Doctor Who would be continuing to another thirteen-episode season in 2006.

February 3rd and 5th were spent at Doctor Who's regular studio home of Unit Q2 in Newport, for segments in the TARDIS. In between, on the 4th, more City Hall sequences were taped at Glamorgan House. February 8th saw Ahearne's team return to Mermaid Quay, with the Bosphorus Restaurant appearing in the scene where the Doctor spotted Margaret's photo in the newspaper. The 10th was again devoted to the standing TARDIS set at Unit Q2, after which cast and crew returned to Glamorgan House on February 11th and 14th. Work on the 15th began with Mickey's arrival at Cardiff Railway Station, before moving back to Mermaid Quay. Finally, insert shots were recorded at Unit Q2 on February 18th.

Davies continued to keep an eye out for actors he wanted to work with again, and he was delighted by Aled Pedrick's performance in the small role of the mayor's secretary, Idris Hopper. Davies hoped to make Idris a regular character in Torchwood, only to learn that Pedrick was unavailable. Instead he created Ianto Jones, played by Gareth David-Lloyd, to take Idris' place.

• Doctor Who Magazine Special Edition #11, 31st August 2005, “Fact File: Boom Town” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #50, 2018, “Story 165: Boom Town”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Inside Story by Gary Russell (2006), BBC Books.