Project Inferno is designed to drill down through the Earth's core, where it will release a powerful new energy source called Stahlman's Gas, named after the project's director. But the Doctor realises that unleashing Stahlman's Gas will have horrible consequences for the planet, and indeed his fears are confirmed when a substance oozing up from the drill shaft begins mutating men into bestial monsters. Before the Doctor can do anything to stop Stahlman, however, a power surge in the TARDIS console sends him to a hostile parallel universe where Project Inferno is nearing completion.

The story originally intended to conclude Doctor Who's seventh season was Brian Wright's “The Mists Of Madness”, which was commissioned by script editor Terrance Dicks in February 1969. Some months later, however, Wright accepted an academic writing post in Bristol, leaving him no time to complete his Doctor Who assignment. Consequently, Season Seven's final slot was still empty when Barry Letts became the programme's producer in October. Around this time, potential options to fill it included “The Shadow People” by Charlotte and Dennis Plimmer, and “The Vampire Planet” by William Emms.

However, Dicks suggested approaching Don Houghton, who had been the script editor on Crossroads when Dicks had written for the soap opera. Houghton had also worked with Letts on Emergency -- Ward 10. For Doctor Who, Houghton found inspiration in a scientific article he had read several years earlier, concerning an American proposal to drill more than five kilometres through the Earth's mantle in the Pacific Ocean to the Mohorovičić discontinuity, the boundary between the planet's mantle and its crust. Since Houghton's story would have to carry seven episodes, Letts and Dicks suggested a subplot involving the Doctor being cast into a parallel universe. A storyline for “The Mo-Hole Project” -- taking its name from that of the American mission -- was commissioned on November 27th. Oddly, assistant script editor Robin Squire also developed an untitled idea for a Doctor Who story involving a drilling project and alternate timelines, apparently around the same time.

Don Houghton was asked to add a monster to the storyline, resulting in the inclusion of the Primords

The storyline bore the title “Operation: Mole-Bore” by the time Houghton delivered it in early December, and this became simply “The Mole-Bore” soon after the full scripts were commissioned. This formally took place on January 9th, 1970 although, by that time, Houghton had already started writing them. A major change requested by Letts and Dicks was the addition of some sort of monster to the proceedings, resulting in the inclusion of the Primords (initially spelt “Primeords”). One element removed from the storyline was the Doctor returning to the normal world while in possession of the missing microcircuit from the parallel universe's computer, and then using it to repair Stahlman's sabotage. Two name changes included Sir Keith Mulvaney becoming Sir Keith Gold (after the surname Rose was also considered), and Private Peters becoming Private Wyatt. Houghton was also asked to introduce a type of alien karate for the Doctor to use in hand-to-hand combat; this was originally called Feltian karate before being renamed Venusian karate.

The director assigned to “The Mole-Bore” was Douglas Camfield, whose last Doctor Who work had been on The Invasion a year earlier. It was Camfield's decision to have the Primords appear as werewolves rather than ape-creatures, as Letts had envisaged them. In the role of Petra Williams, Camfield cast his wife, Sheila Dunn, after Hammer horror actress Kate O'Mara -- later the Time Lord villainess called the Rani -- proved unavailable. Camfield also hired John Levene to play a United Nations Intelligence Taskforce (UNIT) sergeant who had gone unnamed in Houghton's scripts. Levene had played UNIT Corporal Benton in The Invasion and so it was decided that this should be the same character, having been promoted between appearances. Levene was contracted for “The Mole-Bore” on February 23rd, and continuity was further maintained via the character's last-minute inclusion in the preceding serial, The Ambassadors Of Death.

In mid-February, Houghton's serial was retitled again, this time to “Project Inferno”. Meanwhile, discussions were under way about the future of Doctor Who. Letts and Dicks had been sufficiently concerned about the possibility of cancellation that each had been pursuing replacement projects -- a show about a displaced Australian cowboy called Snowy White for Letts and a sitcom entitled Better Late for Dicks. Around the end of the month, however, BBC Head of Drama Shaun Sutton and Ronnie Marsh, who had succeeded Sutton as Head of Serials, confirmed that the early ratings for Jon Pertwee's first year as the Doctor had been sufficient to earn a second season.

With the renewal in hand, one change Letts was determined to make was to drop the Liz Shaw character, whom he felt was too sophisticated to serve as a sidekick. As it transpired, actress Caroline John was now pregnant, and hence unlikely to be available for Season Eight. Nonetheless, John was very hurt by the decision to drop her from Doctor Who and, for many years thereafter, she laboured under the misapprehension that it was a commentary on her acting ability.

Location filming for “Project Inferno” began on March 31st. The lone venue used for the serial was Berry Wiggins & Co, an oil refinery situated at Hoo in Kent, which served as the Inferno drill site in both the regular and parallel universes -- referred to as “Warp I” and “Warp II” in Houghton's scripts. Work continued there until April 3rd, when stuntman Roy Scammell set a world record by performing Wyatt's fifty-foot fall from a gantry. Unfortunately, this day was marred when Pertwee, driving Bessie, accidentally hit stuntman Alan Chuntz while recording the Warp II scene where the Doctor tried to evade the parallel-world soldiers. Despite needing eighteen stitches, Chuntz immediately returned to work in order to lift Pertwee's spirits. Nonetheless, the delay meant that some Episode One scenes -- of Benton and other UNIT troops searching for Slocum, and of the Primord murdering a UNIT soldier named Collins -- had to be abandoned.

From April 6th to 8th, work on “Project Inferno” proceeded at the BBC Television Film Studios in Ealing, London. The first day dealt with the Doctor's journey between universes -- through what the scripts called the “Nightmare Warp”. The middle day saw the completion of the model sequences, while the last day focussed on the transformations of both Platoon Under Leader Benton and Professor Stahlman into Primords. Around this time, the serial's title was truncated to simply Inferno.

As he had done to a more limited extent on The Silurians earlier in the season, Letts decided to experiment with the recording schedule for Inferno. Normally, Doctor Who would have one studio day each week, which would be devoted to a single episode. However, Letts thought it would be more efficient to record two episodes over two consecutive days on a fortnightly basis. This way, sets could be left up overnight, reducing the wear and tear which came from having them taken down and set back up every week. As such, Inferno was made in four distinct recording sessions, rather than seven.

When Jon Pertwee would not follow directions, an irate Douglas Camfield descended to the studio floor

The first such session covered April 23rd and 24th, and was devoted to Episodes One and Two. However, because so much of these installments had been completed on film, Camfield elected to use the first day entirely for camera rehearsals, confining recording to the Friday. Unfortunately, things did not go well on this day. Pertwee would not follow Camfield's directions and, when the star argued with production assistant Chris D'Oyly John, an irate Camfield descended to the studio floor, forcing Sheila Dunn to intervene and persuade Pertwee to comply.

Things would come to a head during rehearsals for the second studio session at the start of the following week. By this time, Caroline John was also unhappy because Nicholas Courtney had convinced Camfield and Letts that it should be the Brigade Leader who interrogated the Doctor in Episode Three rather than Section Leader Shaw. As tensions mounted on April 27th, Camfield collapsed, prompting D'Oyly John to summon Letts. Dunn revealed that her husband was suffering from a heart murmur, and it was agreed that Camfield should be removed from Inferno to safeguard his health. Letts' first inclination was to hire a new director to finish the serial but, because Camfield had already completed much of the planning for the final five episodes, Letts decided to take over the story himself. He had previously directed The Enemy Of The World in 1967.

With Letts now in charge, recording resumed on May 7th and 8th. Work on these days concentrated on Episodes Three and Four, as well as the Warp I material from Episode Six. The sinister man depicted on the Warp II propaganda posters was actually Jack Kine, the head of Special Effects. Episode Five was taped alongside the remainder of the sixth installment over May 21st and 22nd. These were the only days during which recording took place in Studio 6 at BBC Television Centre in White City, London; the remainder of Inferno was taped in TC3 instead. Pertwee had a rare opportunity to play a second role during this session, as he pre-recorded the announcer's voice -- performed in imitation of Nazi propagandist William Joyce, known as “Lord Haw-Haw” -- heard over the Brigade Leader's radio. Cast and crew then returned to TC3 for the final episode, which was taped on May 29th. This day marked the end of both the production block as a whole, and Caroline John's time on Doctor Who.

In post-production, Letts became concerned that Pertwee's voice was too recognisable as the radio announcer. As such, he removed this segment from Episode Five, although it was inadvertently retained on the prints distributed internationally. Inferno had the distinction of being the final Doctor Who serial to rely solely on stock tracks for its incidental music; this had been done periodically throughout the programme's history to date, typically as a cost-saving manoeuvre.

Inferno Episode Five, broadcast on June 6th, was the last Doctor Who episode to be followed by the short-lived American sitcom The Debbie Reynolds Show. Episode Six was scheduled ten minutes later than usual, at 5.25pm, to accommodate the broadcast of the Wightman Cup tennis tournament. As a result, it led directly into a news update and then Dad's Army. Then, on June 20th, the Season Seven finale was followed by the last match of the Television Top Of The Form quiz show tournament, pitting secondary students from Salisbury against their counterparts from Inverness.

• Doctor Who Magazine #305, 27th June 2001, “Archive: Inferno” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Instant Karma” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #16, 2018, “Story 54: Inferno”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.