The Doctor agrees to transmat down to Earth to make sure everything is okay before the Nerva survivors begin to reclaim their planet. There, he, Sarah Jane and Harry discover the presence of a Sontaran named Styre, who is performing cruel experiments on a band of captive humans. Styre's goal is to discover the weaknesses of the human body -- weaknesses the Sontarans will then exploit in their quest to dominate the universe.

Since 1971, a Doctor Who season had been comprised almost exclusively of four- and six-part stories. As he developed the outline of the programme's twelfth recording block, however, script editor Robert Holmes decided to try a different approach. He was not a fan of lengthy stories, believing that most such adventures were excessively padded. So, as an experiment, Holmes elected to replace what would normally be a six-part story with two serials -- one four episodes long, the other two episodes long.

In so doing, Holmes had to be careful not to put additional strain on Doctor Who's schedule and budget. Rather than assign both location and studio time to each of the two new adventures, Holmes decided that one would be made entirely on location, while the other would be confined exclusively to the studio. In this manner, the two serials could effectively function as a single production, just like the six-part story they were supplanting.

Already commissioned at this stage was a suitable four-part adventure: “Space Station” by Christopher Langley. For the shorter story, Holmes approached the writing team of Bob Baker and Dave Martin; the duo had most recently handled The Three Doctors two years earlier. Holmes gave Baker and Martin a general idea of the storyline he wanted, including its setting of a far-future, abandoned Earth and the return of the Sontarans, a race Holmes himself had introduced in The Time Warrior the previous year. Although Holmes was generally adverse to bringing back old monsters, he saw this as a way to capitalise on the cost of the Sontaran costume and the spaceship exterior prop fabricated for the earlier adventure; hence Baker and Martin were told to write with the constraint that only one Sontaran costume would be available. (The authors gave their Sontaran the name Weam Styre, although in the event he would only be referred to as Styre.)

Baker and Martin were commissioned to write a storyline for “The Destructors” on May 23rd. The script for the first episode was requested very shortly thereafter, on June 5th, while the commission for part two following on July 6th. By this time, “Space Station” had been abandoned, and “The Destructors” was paired with a new story called The Ark In Space.

From the start, Baker and Martin envisioned filming their serial in the West Country. 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, by having the top of Nelson's column poking up out of the ground in homage to the 1968 post-apocalyptic film Planet Of The Apes. Much of the action of their scripts was set in the ruins of an old priory, and so many of Styre's torture devices were medieval in nature. Holmes rewrote episode two extensively, in the process removing a subplot in which it would have been revealed that Styre was controlling Vural's mind.

Brought on board to direct both The Ark In Space and “The Destructors” was Rodney Bennett. Bennett had begun his career as a producer for radio before shifting to television when BBC2 was created. He went on to direct episodes of programmes such as Z Cars and Thirty Minute Theatre. At the suggestion of production unit manager George Gallaccio, Bennett decided to record “The Destructors” using the BBC's new outside broadcast (OB) videotape facilities rather than the traditional film, in order to make editing more convenient. However, a limitation of OB work was that colour separation overlay -- an effect often called into service on Doctor Who -- could not be employed. Styre's robot had originally been envisioned as a CSO creation, but it would instead have to be realised as a practical prop.

Around September 18th, Holmes and producer Philip Hinchcliffe decided to change the title of the story -- which been given the production code Serial 4B -- to The Sontaran Experiment. This disappointed Baker and Martin, who felt that it ruined their only cliffhanger. Ironically, despite Holmes' intentions, a new, more lightweight costume was created for Kevin Lindsay to wear as Styre. Lindsay had also played Linx in The Time Warrior, and had found the costume very demanding to wear. (This was exacerbated by Lindsay's heart condition, an ailment which would result in his death just six months later.)

Although Bennett concurred with Baker and Martin's suggestion that The Sontaran Experiment should be recorded in the West Country, scouting revealed that no suitable site could be located for the authors' intended priory setting. Holmes therefore rewrote the action to take advantage of Dartmoor's natural rock formations. Taping began with three days near Postbridge from September 26th to 28th, for scenes set around the transmat, the humans' camp, and the pit. Amongst the guest cast was Glyn Jones, who had previously been involved with Doctor Who in a completely different capacity, writing The Space Museum in 1965.

Recording then shifted to Hound Tor near Manaton beginning on the 29th, the locale for material in the vicinity of Styre's craft. Late on the first day, however, Tom Baker slipped on a patch of wet grass while enacting the Doctor's confrontation with Styre before a captive Sarah. Recording stopped (forcing the abandonment of a sequence in which the Doctor and Harry meet and hear Sarah Jane screaming) and Baker had to be carried back to the nearest car and driven to the hospital. There, it was discovered that he had cracked his collarbone. Nonetheless, despite his discomfort, Baker was able to finish shooting The Sontaran Experiment. He was able to conceal a neck brace beneath his scarf and coat, although his performance was restricted as he was forced to remain very still. Work at Hound Tor continued until October 2nd, marking the conclusion of work on Doctor Who's first two-part story since The Rescue in 1964.

• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 0 426 20369 8.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• 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.
• In-Vision #3, March 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.