In the 26th century, the Doctor, Adric, Nyssa and Tegan come to the aid of a platoon of soldiers, which is investigating the murder of a scientific team in a cave complex on Earth. The Doctor discovers that the killers are actually androids serving the Cybermen, and are guarding a bomb intended to destroy the planet. The Doctor disarms the explosive but by tracing the detonation signal, he learns that the greatest danger is yet to come. The Cybermen have secreted themselves on board a freighter heading for Earth, which will unknowingly serve as the bridgehead for a massive invasion.

The story originally scheduled to be the penultimate adventure of Doctor Who's nineteenth season was “The Enemy Within” by well-regarded science-fiction author Christopher Priest. A storyline for this serial was commissioned by script editor Christopher H Bidmead on December 5th, 1980. Shortly thereafter, Bidmead left Doctor Who and was replaced on an interim basis by Antony Root, who requested full scripts for “The Enemy Within” on February 6th, 1981.

Around this time, producer John Nathan-Turner decided to write Adric out of Doctor Who: the character was now viewed as unsuccessful, and actor Matthew Waterhouse had been a sometimes awkward presence on set. Nathan-Turner felt that Adric did not lend himself to a traditional departure scenario, however, and instead decided that he should be killed off. Not only would this accrue a lot of publicity for Doctor Who -- no regular character had exited the programme in that manner since short-lived companions Katarina and Sara Kingdom in Season Three's The Daleks' Master Plan -- but Nathan-Turner also thought that it would imbue Doctor Who with a greater sense of danger and excitement, while making the surviving characters much less invulnerable than had previously been the case.

It was decided that “The Enemy Within” should be the story in which Adric met his end; Priest duly amended his plot and completed his scripts. In mid-June, however, a serious disagreement arose between Priest and Nathan-Turner over payments for rewrites. Following a venomous telephone conversation, it became clear that “The Enemy Within” would have the be withdrawn from the schedule and replaced with a new story.

By now, Root's three-month tenure on Doctor Who had come to an end, and he had been replaced as script editor by another trainee named Eric Saward. It had originally been anticipated that Saward would merely fill the gap before Root could come back to Doctor Who, but by June it had become clear that Root would not be returning. Instead, the decision was made to install Saward as the programme's permanent script editor. Under BBC regulations, this meant that Saward was now forbidden from writing new Doctor Who serials, but he and Nathan-Turner nonetheless agreed that he would provide the replacement for “The Enemy Within”. This was made possible by the expiry of Saward's initial three-month contract towards the end of June; consequently, permission was sought for Saward to write a story called “Sentinel” on June 29th, just before his new long-term contract came into effect. To further avoid any appearance of impropriety, Root agreed to perform minor work on “Sentinel” so that he could be credited as the script editor.

Both Saward and Peter Davison were fans of the Cybermen, a monster which had featured in Doctor Who only once since 1968 (in 1975's Revenge Of The Cybermen). Urged on by fan advisor Ian Levine, Nathan-Turner was eager to bring back popular monsters from the series' past, and readily agreed to the return of the Cybermen in “Sentinel”. Tired of Doctor Who monsters always being killed off at the end of the first appearance, Saward considered having the Cyber Leader survive “Sentinel” -- providing yet another sign of the Fifth Doctor's fallibility in comparison to his predecessor -- but ultimately decided against it. Meanwhile, it fell to Nathan-Turner to inform Waterhouse of his character's fate. The young actor was initially appalled by the decision, because Adric's death meant that he would not be able to return to Doctor Who at a later date. He was mollified to an extent, however, when the producer pointed out that the Doctor could always encounter Adric at a time prior to his demise.

Costume designer Dinah Collin was assigned the task of bringing the Cybermen into the Eighties, and worked on the project with Richard Gregory of effects firm Imagineering. They decided to abandon the rubber diving suits which had previously been the basis of the Cyberman outfit, opting instead of the more high-tech look of army G-suits. At Nathan-Turner's suggestion, the jaws of the updated Cybermen were left clear so that the actors' mouths could be seen; the producer felt that this would reinforce the notion that the Cybermen had once been human. In a similar vein, Collin and Gregory considered leaving the Cybermen's hands bare -- as had been the case in their first appearance, in 1966's The Tenth Planet -- and then seamlessly integrating the flesh with the Cyberman's “metallic” arm. However, it was ultimately decided that this effect would be too complex to achieve. Collin also wanted to do away with the “handlebars” on the sides of the Cyberman helmets, but this was vetoed by Nathan-Turner and Saward, who felt that they were an essential part of the Cyberman image.

During October, “Sentinel” -- designated Serial 6B -- was retitled Earthshock. Its director was Peter Grimwade, who had just completed Kinda. Grimwade, Saward and Nathan-Turner were all keen that Earthshock should try, as much as possible, to capture the fast-paced feel of a feature film. As a result, Grimwade's final camera scripts were extremely lengthy, with part four alone running to eighty-nine scenes: far more than was normal for Doctor Who at that time.

Work on Earthshock began with one day of location filming at Springwell Lock Quarry in Rickmansworth, Hertfordshire, for the exteriors of the cave complex. Studio recording then comprised two three-day sessions in BBC Television Centre Studio 8. The first of these took place from November 10th to 12th. The opening day dealt with most of the TARDIS interior scenes. Material in the main cavern and the various tunnels was the focus on the 11th, together with some of the sequences in the small cavern. These were completed on the 12th, along with the awakening of the dormant Cybermen and the climactic TARDIS scenes.

Also recorded on November 12th was all of the action in Cyber Control, which included flashbacks to older Doctor Who stories. Nathan-Turner had been delighted by the reception of a similar sequence in the concluding episode of Logopolis the previous year, and so was keen to include more such moments in Season Nineteen. Earthshock was an obvious candidate, and Saward and Levine worked together to select one clip for each Doctor who had previously encountered the Cybermen. They settled on extracts from The Tenth Planet (the First Doctor), The Wheel In Space (the Second Doctor) and Revenge Of The Cybermen (the Fourth Doctor). The latter segment was converted to monochrome to preserve continuity with the first two clips.

The second studio block spanned November 24th to 26th, and included all of the scenes set aboard the freighter. Although Adric's death was amongst the material filmed, Waterhouse's time on Doctor Who was not yet over, as the actor was required to appear as an ersatz Adric in the season's concluding serial, Time-Flight. Model shots were also taped on the final day; the freighter mock-up included elements which were an intentional homage to the Nostromo from the 1979 feature film Alien. By this stage, it had been decided that Earthshock part four would conclude with the credits running silently over a shot of Adric's shattered badge for mathematical excellence; Nathan-Turner had borrowed this idea from a broadcast of the long-running soap opera Coronation Street. This would mark the only occasion in the programme's history in which a Doctor Who episode would not end with the traditional theme tune.

In post-production, Grimwade became very unhappy with the incidental music composed by Malcolm Clarke, which largely relied on natural metallic sounds such as hammers striking girders. The director complained to Nathan-Turner about the score, but because no time remained for a replacement to be created, it was decided to retain Clarke's composition. Meanwhile, the BBC's listings magazine Radio Times offered Nathan-Turner a cover to promote the return of the Cybermen. The producer was keen to keep the monsters (and Adric's fate) a secret, however, and declined the invitation, despite the fact that Doctor Who had not appeared on a Radio Times cover since 1973.

The broadcast of Earthshock episode four brought Antony Root's connection with Doctor Who to a belated close. Root went on to script edit The Chinese Detective before leaving the BBC. He subsequently became a producer, earning credits on programmes such as Cold Comfort Farm, The Grand, and the American remake of Touching Evil, as well as the 1991 movie Edward II. Root also served as Head of Drama at Thames Television.

• Doctor Who: The Handbook: The Fifth Doctor by David J Howe and Stephen James Walker (1995), Virgin Publishing, ISBN 0 426 20458 9.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing, ISBN 1 85227 680 0.
• Doctor Who Magazine #239, 5th June 1996, “Archive: Earthshock” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #1, 2001, “Prince Charming” by Andrew Pixley, Panini Publishing Ltd.
• In-Vision #60, September 1995, “Production” edited by Anthony Brown, Cybermark Services.