Once upon a time, the Doctor helped broker an agreement which led to the secret resettlement of twenty million Zygons to modern-day Earth. Now, however, the truce is unravelling as a splinter faction of Zygons calling itself Truth or Consequences demands the right to live without taking human form. The Doctor is summoned back to Earth by Osgood, just before she is captured by the terrorists. Matters quickly escalate as the Zygon High Command is kidnapped and killed. And even the Doctor may not suspect the extent of the splinter group's success in infiltrating his circle of friends and allies...

When Doctor Who executive producer Steven Moffat resurrected the Zygons for 2013's The Day Of The Doctor, he was already anticipating their return in a subsequent season. With the Zygons having engaged in a relatively straightforward invasion narrative for their debut appearance in 1975's Terror Of The Zygons, Moffat now saw their shapeshifting abilities as an opportunity to tell a very different kind of story. He imagined what would happen if a colony of Zygons was revealed to be living incognito on present-day Earth. This would allow an exploration of the kind of paranoia which, during the twentieth century, had been associated with phenomena such as the Red Scare -- the Western belief that both regular individuals and government officials were being indoctrinated as Communist sympathisers. More recently, it had manifested in events like the wave of Islamophobia that arose following the terrorist attacks against the United States on September 11th, 2001.

Moffat decided to incorporate elements into The Day Of The Doctor which would set the scene for the Zygon story to come. As such, Kate Stewart of the Unified Intelligence Taskforce (UNIT) would be left with the responsibility to police a peace treaty with a colony of dispossessed Zygons while her scientific officer, Osgood, would forge a friendly relationship with her own Zygon duplicate. Having established these starting points, however, Moffat sought another writer to tackle the two-part adventure he envisaged for Doctor Who's thirty-fifth season. Impressed with the emotional discourse between the Doctor and Clara in Season Thirty-Four's Kill The Moon, Moffat offered the script to its author, Peter Harness, during a press screening for that year's premiere episode, Deep Breath, on August 7th, 2014.

The title of Episode 1 was a reference to 1956's Invasion of The Body Snatchers, which was itself inspired by the Red Scare

Harness accepted Moffat's offer, and he adopted the executive producer's preferred approach of structuring the two parts of the story to be notably different in style. The opening installment was called “Invasion Of The Zygons”, in reference to the 1956 horror film Invasion Of The Body Snatchers, which was itself inspired by the Red Scare. It would have a broad, multi-continental scope and would contrast sharply with the more personal focus of the concluding episode, “Truth Or Consequences”, which would explore the struggles of a Zygon who had taken Clara's form.

Although he enjoyed the opportunity to tackle such sophisticated ideas and introduce an unusually gritty tone to Doctor Who, Harness was also mindful of the need to appeal to the younger members of the audience. As such, he decided that several of the key Zygons would be masquerading as children, with their base of operations located at a school. Harness also wanted to reintroduce Courtney Woods, the Coal Hill School student played by Ellis George who had played an important role in Kill The Moon.

The first draft of “Invasion Of The Zygons” was completed around the start of February 2015. At this stage, much of the action in the first episode took place in Azerbaijan, a real country on the Caspian Sea between Russia and Turkiye, as opposed to the fictional Turmezistan. Osgood had been taken there after her capture by the radicalised Zygons in a raid on a UNIT safe house in London. The leader of the UNIT forces in Azerbaijan was General Bambera; the character's name recalled the Brigadier who had appeared in 1989's Battlefield. Amongst Kate's personnel in London was McGillop, the character played by Jonjo O'Neill in The Day Of The Doctor, and Carol Bell, who was presumably meant to be the same Corporal Bell played by Fernanda Marlowe in two 1971 serials, and who was given the first name Carol in Gary Russell's The Scales Of Injustice, a 1996 novel in Virgin Publishing's Doctor Who: The Missing Adventures range.

In Truth or Consequences -- a real New Mexico town which changed its name from Hot Springs in 1950 in response to a radio quiz show promotion -- Kate learned of a missing police officer named Bonnie Carter, who was identical in appearance to Clara and had departed with the Zygons. The first death amongst the Zygon community in Truth or Consequences was called Clyde Orson”, and was a duplicate of Clara's late boyfriend, Danny Pink. Danny had been killed in the Season Thirty-Four finale, Death In Heaven, but apparently had a future descendant named Orson in the same year's Listen.

In the first draft script, the failed UNIT attack on the Zygon hatchery left Clara on the brink of death

One of the leaders of the Zygon splinter cell was called Blinovitch, and had taken the form of Courtney Woods; in early drafts of Kill The Moon, Blinovitch was revealed to be the surname Courtney would use as an adult. In the late stages of “Invasion Of The Zygons”, Blinovitch was the captive Zygon taken aboard the presidential aeroplane. At one point, she escaped her bonds and attacked the Doctor, only to be stopped by Osgood, who revealed herself to be a Zygon as well. This would have confirmed that the Osgood murdered by Missy in Death In Heaven was the human original. The radicalised Zygons were using London Underground trains to ferry their human captives, rather than lifts. The failed UNIT attack on the Zygon hatchery left the real Clara hospitalised and on the brink of death after being shot.

Harness finished his initial draft of “Truth Or Consequences” in early March. The Doctor and Osgood used the TARDIS to travel to the past and follow the Zygons as they resettled in New Mexico. They learned that the Zygon leaders -- the schoolgirls Claudette and Jemima -- had executed Clyde Orson because the death of Danny Pink had caused him to believe himself to actually be the original human; this was their attempt to preserve the peace treaty. However, when Bonnie was called to investigate the murder, it exacerbated the tension she was already sensing between the Americans and the “British” expatriates, leading to her radicalisation.

In the present day, three Doctors were active simultaneously -- two of them being Zygon allies -- while Kate was rescued from Truth or Consequences by American marines. The United Kingdom was being isolated by the international community as it became increasingly dominated by the Zygons; scenes would have depicted politicians and even the cast of the soap opera EastEnders engaged in replacing authentic humans with Zygon duplicates. However, the Doctor discovered that the splinter Zygons were being manipulated by a human cabal who intended to wipe them out and take over Great Britain. He brought a recalcitrant Bonnie and the United Nations Security Council together to forge a lasting peace treaty. Bonnie then sacrificed her physical form, allowing Clara to take control of it as her real body succumbed to its injuries.

Later drafts of “Invasion Of The Zygons” replaced McGillop and Bell with Jac, a character whom Moffat had introduced in the season premiere, The Magician's Apprentice / The Witch's Familiar. She had been designed to fill the role of Osgood's apparent successor, to further the pretence that Osgood's story had ended with her murder in Death In Heaven. Meanwhile, General Bambera became Colonel Walsh, the notion of the Osgood Boxes was added, and the Zygons no longer made use of foot-long caterpillar-like creatures.

Clara's efforts to influence Bonnie were originally unsuccessful, leaving Kate with no choice but to shoot her

By the start of April, the second episode had been renamed “Inversion Of The Zygons”. Harness had now jettisoned the trip back in time, which prompted him to abandon the duplicates of Courtney Woods and Danny Pink as well. The multiple Doctors were also dropped, while Clara's efforts to influence Bonnie became a significant element of the narrative. Originally, these attempts were ultimately unsuccessful, leaving Kate with no choice but to shoot Bonnie to stop her from triggering an Osgood Box. The Doctor then revealed that this was not the first time that she had been forced to take lethal action towards a radicalised Zygon, only to forget the events due to the memory-wiping effects of the Black Archive.

Harness had also observed that Doctor Who had only occasionally ventured into the realm of body horror, with 2011's The Rebel Flesh / The Almost People representing a rare exception. As such, he became interested in exploring how people would react if a Zygon were forced to assume its natural form in their midst. This material originally focussed on a vicar called Emily who transformed in the middle of a police officer's funeral, before the ill-fated Etoine was developed instead.

“Invasion Of The Zygons” and “Inversion Of The Zygons” were Season Thirty-Five's seventh and eighth episodes in production order and, as a result, they formed Block Four. They would occupy the same positions on transmission, although some thought had been given to scheduling Face The Raven first, before its narrative was altered to include Clara's apparent demise. Directing the Zygon adventure was Daniel Nettheim, who had been a fan of Doctor Who during his childhood in Australia, and who had made several 2010 episodes of the unofficial spin-off series K·9 in his home country. Viewing the story as an urban thriller, Nettheim looked to movies like 2004's The Bourne Supremacy for visual inspiration. Amongst his cast were Cleopatra and Sasha Dickens, playing Claudette and Jemima. Although the characters had originally been envisaged as twins, it had now been decided that having sisters play the role would convey a suitable sense of association.

Production began with a week on location in Cardiff. The first day of work was May 5th, when the Old Custom House posed as the UNIT safe house. The safe itself was hidden behind a picture of the First Doctor from 1966's The Celestial Toymaker; in the script, however, the portrait was of Sergeant Osgood, a member of UNIT in 1971's The Daemons, who was implied to be the father of the modern character. The video footage of the two Osgoods was also recorded at the same venue. One Osgood wore a variation on the Seventh Doctor's question-mark pullover, as Harness had requested. However, the other Osgood paired the Fourth Doctor's lengthy scarf with a bow tie of the type worn by the Eleventh Doctor, as opposed to the writer's suggested attire of the Fifth Doctor's vest with a sprig of celery. More safe house scenes were recorded at the Old Custom House on May 6th and 7th, with part of the latter day also spent at Pennsylvania, an apartment block which was suitable as Etoine's residence. The retail complex where Etoine met his end was actually the Maelfa Shopping Centre, where cameras were rolling on the 8th and 9th.

The closing material in which Bonnie adopted Osgood's form was only added partway through filming

Following a day off on Sunday the 10th, Nettheim's team spent May 11th at Lydstep Flats in Cardiff, which again provided the exterior and common spaces of Clara's apartment block, as it had in 2013's The Time Of The Doctor. Footage of the Doctor and Osgood in the van was also shot at nearby Western Avenue. Work on the 12th began at the Barry Shooting Range in Barry, where the Zygon tunnels were staged. Cast and crew then shifted to the premises of eCubed Solutions at MOD St Athan, for sequences outside the presidential aeroplane. Only at this stage was the script modified to include the closing material in which Bonnie adopted Osgood's form. The first studio days for the serial were May 13th and 14th, at Doctor Who's usual production home of Roath Lock Studios in Cardiff. The main concern was action in Clara's flat, although Nettheim also recorded the concluding TARDIS scene on the second day. It was back to Barry on the 15th, where Watchtower Bay was the site of the beach to which the Doctor and Osgood parachuted.

After the weekend, the remaining beach sequences were taped at Watchtower Bay on May 18th. The same day, shots of Bonnie atop the White Cliffs of Dover were captured at the nearby Nell's Point. The same area had been the site of the holiday camp where 1987's Delta And The Bannermen had been filmed; its demolition had been completed in 2005. From May 19th to 21st, material in the Black Archive was recorded at Roath Lock, with work on the middle day focussing on the Doctor's crucial speech to Kate and Bonnie in “Inversion Of The Zygons”. Nettheim and Peter Capaldi had discussed their approach to this scene in detail, and had ultimately agreed that it should be filmed as a long single take. In the event, it was recorded only twice, with most of the televised footage derived from the initial version. Part of the 21st was spent on the set for the little boy's apartment, and Nettheim also captured several inserts.

The last day of production for the week was May 22nd, during which more activity at Roath Lock saw a second unit continue to work on a variety of pick-up shots. However, the venue for Nettheim's main unit was Canal Park in Cardiff, with the Doctor meeting Claudette and Jemima at its playground. Unfortunately, during the sequence of the Doctor bolting for the TARDIS after their abduction, Capaldi injured his knee. Since the tear to his meniscus would require surgery and recuperation time, it was agreed that filming for both the story and the season would proceed as scheduled, with the production team making any necessary accommodations to ensure that the actor did not sustain further injury. Meanwhile, the shot of the two Osgoods walking away together was also taped at Canal Park; the duffel-coated attire worn by the duplicate who had previously been Bonnie was inspired by the Seventh Doctor's outfit in 1989's The Curse Of Fenric.

A weekend off gave Capaldi a chance to rest his knee, after which work on May 25th began back at Canal Park, with the Doctor and Clara bidding farewell to the Osgoods. Other Cardiff locations employed on the busy day included Allensbank Primary School (Claudette and Jemima's junior school), a former music school on Stacey Road (the exterior of the UNIT safe house), and Cathays Cemetery (the site of the original Osgood's grave). Filming at Roath Lock on May 26th and 27th concentrated on scenes in the Zygon cavern, plus further inserts. On the 28th, the Zygon command centre beneath Drakeman Junior School was actually created at Hensol Castle in Hensol; the video message featuring Claudette and Jemima was also taped there. The remaining material in the Zygon cavern was completed at Roath Lock on May 29th, as was footage in the lift at Clara's apartment block.

Following the weekend, Nettheim's team spent June 1st and 2nd at MOD Caerwent. It provided space suitable for the UNIT base in Turmezistan on the first day, and the police station in Truth or Consequences on the second day; an insert of the smoke bomb deployed in the school playground was also captured there on the 2nd. On June 3rd, the FIBUA (Fighting In Built-Up Areas) village at the Sennybridge Training Area in Sennybridge posed as the Turmezistani hamlet, with an ersatz church replacing the scripted hangar as the site of the UNIT massacre. The 4th was a studio day, with Nettheim recording scenes aboard the presidential aeroplane alongside footage of the newsreader and numerous pick-up shots. Work on June 5th began at St Catherine's Church in Cardiff, which served as the interior of the church in Turmezistan. Cast and crew then returned to Roath Lock for the Doctor's rescue of Osgood from beneath the church, plus more effects and insert shots.

It was agreed that the New Mexico material could be shot on Fuerteventura, the second-largest of the Canary Islands

From the outset, Nettheim had been aware that the Doctor Who budget would not permit location filming in New Mexico. However, he and executive producer Brian Minchin still wanted to lend a distinctive look to the footage of the town of Truth or Consequences. In the past year, the programme had twice visited Spain's Canary Islands, off the coast of Morocco. First, Lanzarote had provided the lunar landscape for Kill The Moon and, more recently, Tenerife had been the venue for the surface of Skaro in The Magician's Apprentice / The Witch's Familiar. Now it was agreed that the New Mexico material could be shot on Fuerteventura, the second-largest of the Canary Islands, immediately south-west of Lanzarote. A minimal cast and crew flew to the archipelago on June 6th, with the 7th reserved as the only recording day. Areas of Truth or Consequences were represented by Calle la Cruz in Tiscamanita, while scenes in Kate's and Norlander's vehicles were captured just outside Antigua. With the Zygon story now complete, the Doctor Who team returned home on June 8th.

The two episodes were ultimately broadcast under the slightly modified titles of The Zygon Invasion and The Zygon Inversion. The opening installment was transmitted on Hallowe'en at 8.15pm, five minutes earlier than in recent weeks. By contrast, the concluding episode went out at 8.00pm; in both cases, the changing timeslots were prompted by the dwindling runtime of Doctor Who's lead-in, the dance competition Strictly Come Dancing, as it shed competitors from week to week. For The Zygon Inversion, Doctor Who was followed not by Casualty as usual, but by The National Lottery Live and then the 2015 edition of the Royal British Legion Festival Of Remembrance.

• Doctor Who: The Complete History #82, 2018, “Story 258: The Zygon Invasion / The Zygon Inversion”, edited by Mark Wright, Hachette Partworks Ltd.