With humanity awakening from its long slumber aboard the Nerva Beacon, the time travellers transmat down to Earth to make sure the matter transmitter is fully operational. While Sarah and Harry explore, the Doctor discovers a dead astronaut. Confronted by the man's colleagues, the Doctor learns that they have been under attack ever since they were lured to Earth by a mysterious distress call. Meanwhile, Harry falls into a pit and Sarah seeks help from Roth, another of the astronauts. Together, Sarah and Roth discover that Earth is the setting for a series of cruel experiments devised by a Sontaran named Styre.

The original plan for Doctor Who's twelfth season -- the first for Tom Baker as the Fourth Doctor -- was that it would run for twenty-six episodes, following the convention established over the last three years. With the four-part Robot being held back from the programme's eleventh recording block, this left twenty-two episodes to be recorded between the autumn of 1974 and the spring of 1975, overseen by the new production team of producer Philip Hinchcliffe and script editor Robert Holmes. Although Holmes was wary of six-part serials, which he felt were excessively padded, two were soon lined up for the latter part of the run: Genesis Of The Daleks by Terry Nation and “Loch Ness” by Robert Banks Stewart (which later became Terror Of The Zygons). They joined two four-part stories called “Space Station” by Christopher Langley and Revenge Of The Cybermen by Gerry Davis, leaving two episodes unassigned.

Of these, “Space Station” was intended to be the first to go before the cameras. However, when Langley's scripts were deemed unsuitable around the end of May 1974, Holmes resisted the obvious option to replace it with another six-part adventure. Instead, he decided to develop two linked storylines which could effectively be made as a single production. One would be four episodes long and be confined to the studio; the other would be the programme's first two-part serial since The Rescue in 1965, and would be made entirely on location. Together with a reformatting of Revenge Of The Cybermen to act as a semi-sequel -- utilising many of the same sets -- this would alleviate some of the strain on Doctor Who's often-overburdened budget.

Bob Baker and Dave Martin were told to write with the constraint that only one Sontaran could be seen at a time

With The Ark In Space soon taking shape as the studiobound narrative, Holmes turned to Bob Baker and Dave Martin to provide the shorter adventure. The pair had contributed three serials for Jon Pertwee's Third Doctor -- most recently 1972's The Three Doctors -- and were recommended to Holmes by his predecessor, Terrance Dicks. Holmes asked Baker and Martin for a storyline set on a far-future, abandoned Earth, which would feature the return of the Sontarans, whom Holmes had introduced in 1973's The Time Warrior. Although Holmes was generally averse to bringing back old monsters, he wanted to take advantage of the existence of the costly Sontaran costume and spaceship exterior, which had been fabricated for the earlier adventure. As such, Baker and Martin were told to write with the constraint that only one Sontaran could be seen at a time.

A storyline for “The Destructors” was commissioned on May 23rd. The script for Episode One was requested soon thereafter, on June 5th, while the commission for Episode Two followed on July 6th. From the start, Baker and Martin envisaged the moors and heaths of the West Country as a suitable filming location to bring to life their vision of the wild, deserted Earth. Initially, however, they wanted to incorporate some relics of human civilisation which would imply that “The Destructors” was actually set where London had once stood. For instance, the writers wanted the top of Nelson's column to be found protruding from the ground; this image was inspired by the shot of the Statue of Liberty half-buried on a beach, as seen in the closing moments of the 1968 post-apocalyptic film Planet Of The Apes.

The chief Sontaran in “The Destructors” was referred to as Weam Styre in the scripts. Baker and Martin were inspired by stories of Nazi scientists using concentration camp prisoners as test subjects during World War Two, and the nature of some of Styre's experiments was drawn directly from these reports. Styre's robot had originally been developed for another potential Doctor Who storyline. At one stage, the writers envisaged the automaton as being able to move so fast as to become effectively invisible.

As work on “The Destructors” progressed, Episode Two was reconfigured to give the Doctor a greater physical involvement in confronting Styre. This came at the expense of a subplot in which it would have been revealed that the Sontaran was controlling the mind of Vural, the leader of the GalSec crew. Another late change came when Hinchcliffe decided to simplify the design requirements for “The Destructors”. Originally, the GalSec astronauts operated from a makeshift shelter, while Styre used a ruined priory -- complete with dungeons -- as his base of operations. Instead, the story would be completely set in natural environments. Some medieval aspects of Styre's experiments were eliminated as a result.

Cast as Krans was Glyn Jones, who had written The Space Museum in 1965

Brought on board to direct both The Ark In Space and “The Destructors” was Rodney Bennett. Cast as Krans was Glyn Jones, who had previously been involved with Doctor Who in a completely different capacity, writing The Space Museum in 1965. At the suggestion of production unit manager George Gallaccio, it was decided that “The Destructors” should be recorded using new videotape facilities, rather than on film, in order to expedite the editing process. However, a limitation of the available equipment was that it did not accommodate the chroma key effects technique. This meant that Styre's robot -- originally envisaged as a chroma key creation -- would instead have to be realised as a practical prop.

Despite Holmes' assumption that the costume for the original Sontaran, Linx, would be available for reuse in “The Destructors”, it was discovered that the latex mask and gloves had deteriorated to such an extent that they would have to be replaced. Kevin Lindsay, who had played Linx, agreed to return as Styre, but it was now known that he was dealing with a heart condition and had suffered greatly during the making of The Time Warrior. As such, the new mask was designed to be more lightweight; sadly, however, Lindsay would succumb to a heart attack just six months later.

Shortly before filming began, Baker and Martin were informed that Hinchcliffe and Holmes had decided to rename the story The Sontaran Experiment. The writers were disappointed, because they felt that the new title ruined their only cliffhanger. More happily, it had been decided to record the serial in Devon, in line with Baker and Martin's original conception. Unfortunately, the region was plagued by cool temperatures and damp conditions throughout much of the production schedule . Bennett and his team were joined in Devon by former producer Barry Letts. Although he had now formally left Doctor Who, Letts was still in the process of transitioning to new projects; as such, he decided to make himself available to provide guidance.

The Sontaran Experiment reunited series stars Tom Baker, Elisabeth Sladen and Ian Marter following a hiatus of three and a half months since work wrapped on Robot. After three days of rehearsal on location, recording began on September 26th at Headland Warren Farm in Postbridge. On this first day, Bennett's primary concern was material set at and around the transmat circle. Cast and crew remained at Headland Warren Farm for the next two days, with scenes at the pit taped on the 27th, followed by those at the GalSec encampment on the 28th.

Recording then shifted to Hound Tor near Manaton, the locale for all of the scenes set in the vicinity of Styre's spaceship. Late on September 29th, however, Tom Baker slipped on a patch of wet grass while filming the Doctor's confrontation with Styre at the area where Sarah was restrained. Recording halted immediately, forcing the abandonment of a handful of sequences, including one in which the Doctor found Harry and they heard Sarah screaming. Baker had to be carried back to the nearest car and driven to the hospital, where it was discovered that he had cracked his collarbone.

Despite his discomfort, Baker returned to the production the next day. He was able to conceal a neck brace beneath his scarf and coat, although his performance was severely restricted, as he was forced to remain very still. Stuntman Terry Walsh, who had already recorded scenes as Zake, stood in for Baker to the greatest extent possible, and Baker quickly recognised the value of having an effective and accomplished stunt double. Work at Hound Tor continued until October 1st; the 2nd had been kept in reserve as a stand-by day, but does not appear to have been needed.

• Doctor Who Magazine #237, 10th April 1996, “Archive: The Sontaran Experiment” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #8, 1st September 2004, “You Ain't Seen Nothing Yet” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #22, 2015, “Story 77: The Sontaran Experiment”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• In·Vision #3, March 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.