In 2084, the TARDIS materialises in an underwater seabase, operated by one of two power blocs waging a bitter cold war. It has been infiltrated by enemy agents Solow and Nilson, who are planning to use the brainwashed Maddox to cripple its offensive capabilities. Meanwhile, Commander Vorshak is contending with the appearance of an unidentified object travelling towards the seabase. The Doctor recognises it as a Silurian cruiser. To support their invasion of the seabase, the ancient reptiles have reawakened not only a platoon of their Sea Devil cousins, but also the rampaging terror known as the Myrka.

Having completed work on Season Twenty's Arc Of Infinity in early 1982, Johnny Byrne began developing ideas for Doctor Who's twenty-first season. The directive he was given by the production team was that Season Twenty-One would place a firm emphasis on monsters, in contrast to the previous year's focus on individual villains. Producer John Nathan-Turner was keen to repeat the success of the Cybermen's return in Season Nineteen's Earthshock by mining Doctor Who's past. Script editor Eric Saward was more sceptical of these efforts to appeal to the programme's fans, but he agreed that this would be a suitable approach for the season premiere, which was the slot being eyed for Byrne's contribution. With this in mind, Byrne proposed bringing back the eponymous monsters from 1972's The Sea Devils. The production team was excited by this idea, and suggested that Byrne pair the Sea Devils with their landbound cousins, introduced in 1970's The Silurians.

Byrne began working on his storyline around June 1982. At the time, the Cold War persisted between the United States and the Soviet Union, and Byrne wanted to explore the dynamics of a world in which two supremely powerful blocs sat in opposition, each waiting for the other to pull the trigger. In particular, he was interested in the notion that the people in charge of the weapons might not be reliable in an atmosphere of such heightened tension. In developing his ideas, Byrne drew upon various elements of the science-fiction series Space: 1999, which he had script-edited in the mid-Seventies. In particular, the 1975 episode The Guardian Of Piri by Christopher Penfold had also featured a character who could interface with computers via an implant.

Johnny Byrne had been impressed with Earthshock's kinetic, action-oriented style

Byrne was commissioned to write Warriors Of The Deep on September 10th. Having prepared for the assignment by watching The Silurians and The Sea Devils, he decided that the Silurians should be portrayed as the leaders, while the Sea Devils would operate more in the manner of elite foot soldiers. He asked Saward if he could use Earthshock as a model; Byrne had been impressed with its kinetic, action-oriented style, and wanted to try his hand at a similar type of story. Byrne intentionally avoided identifying his narrative's two power blocs; to this end, he implied a variety of nationalities amongst the seabase personnel.

After delivering the draft scripts for Warriors Of The Deep at the start of 1983, Byrne departed for work in the United States, rendering him unavailable for further rewrites. Unfortunately, Saward subsequently discovered that the episodes were badly mistimed, in some instances running almost double the required length. After consulting with Byrne by telephone, Saward set to work revising the serial by himself; for a brief time, it became known as “Warriors Of The Deeps”. Amongst his deletions were scenes from the point of view of the crew aboard the probe despatched from the seabase, which was destroyed by the Myrka. In the process, Saward addressed criticism levied by Ian Levine; he was functioning as the production office's unofficial fan adviser, and felt that Byrne had not accurately captured the spirit of the classic monsters. It was also Saward who named Tarpok (originally Tanpon) and Scibus.

In the process of rewriting Warriors Of The Deep, Saward made the adventure much more bloodthirsty than Byrne had intended: Icthar, Vorshak and Preston had all survived in Byrne's version, but Saward killed them off in an attempt to portray the brutality of the events. Upon receiving Saward's amendments, Byrne was dismayed by what had become of his serial, although he felt that there was nothing he could do to remedy his concerns. His relationship with the production office cooled considerably from that point onward; a further storyline he was developing called “The Guardians Of Prophecy” was dropped, and Byrne did not return to Doctor Who.

Despite Nathan-Turner's preference for seeking out new directors for Doctor Who, Pennant Roberts was hired for Warriors Of The Deep. Nathan-Turner had consulted Roberts when he was investigating the use of clips from Shada -- Roberts' last Doctor Who story, which had been abandoned due to strike action in 1979 -- for the twentieth-anniversary special The Five Doctors. Nathan-Turner developed confidence in Roberts, and invited him back to Doctor Who. As in the past, Roberts sought to find more roles for women in Warriors Of The Deep, and the result was the casting of Ingrid Pitt as Solow; Byrne had envisaged the character as an elderly male.

The first studio block for Warriors Of The Deep would be brought forward by a week, or else the serial would be cancelled

Roberts initially planned to film the scenes in and around the seabase cooling chamber using the BBC Television Film Studios water tank in Ealing, London. These and other sequences would be recorded from June 15th to 17th, before Warriors Of The Deep entered the studio for a two-day block beginning on June 29th and a three-day session from July 13th. In May, however, Prime Minister Margaret Thatcher called a snap election for June 9th, taking advantage of her popularity in the wake of the Falklands War. The announcement caught BBC Planning by surprise, and facilities had to be swiftly reallocated to accommodate election coverage. Amongst the casualties of this development was Warriors Of The Deep: Nathan-Turner was informed that the first studio block would have to be brought forward by a week, or else the serial would be cancelled.

A labour dispute had resulted in the loss of one story -- Saward's own “The Return” -- from Doctor Who's twentieth season, and Nathan-Turner was determined that Season Twenty-One would not be similarly abbreviated. Left with no other option, the producer agreed to accommodate the new dates. This decision was met with trepidation by the Visual Effects Department, however, with concern raised that the costume for the Myrka could not be completed within the revised schedule. To make matters worse, Mat Irvine, the visual effects designer assigned to Warriors Of The Deep, would be delayed by his commitments to The Odd Job Man, which had fallen behind schedule. Adding to the chaos was the realisation that filming at Ealing would no longer be possible, both because the crew would be required to cover the election and because of scheduled maintenance to repair a leak in the water tank. No alternative dates were available, which meant that Roberts would have to make other arrangements at very short notice.

Meanwhile, by the end of May, Nathan-Turner and Saward had largely finalised their plan to deal with the various cast changes which would be sweeping through Doctor Who during Season Twenty-One. It was agreed that the departures of Tegan and Turlough would be spread out around the middle of the year. When Mark Strickson was contracted on February 8th, it was only for twenty episodes, and Janet Fielding would likewise leave partway through the season. Unusually, the Fifth Doctor would regenerate in the penultimate story of 1984. This would give the viewing audience an opportunity to acquaint themselves with the new TARDIS crew before the programme went off the air for nine months. Peter Davison was unhappy with this approach: he had signed a contract for the entirety of Season Twenty-One in November 1982, and was frustrated that he would not be able to complete three full years on Doctor Who. The only other occasion on which a Doctor had been replaced mid-season was back in 1966, when the transition from William Hartnell to Patrick Troughton had been prompted by the deteriorating state of Hartnell's health.

John Nathan-Turner's only choice for the Sixth Doctor was Colin Baker

Nathan-Turner's first and only choice to portray the Sixth Doctor was Colin Baker. They had originally met when Baker played Commander Maxil in Arc Of Infinity. At the time, Baker had been resigned to the fact that this would put him out of the running to play the Doctor some day. He had enjoyed Doctor Who since its premiere in 1963, and had previously made inquiries about the lead role prior to Davison's casting in 1980.

However, Nathan-Turner had been taken with Baker's humour and charisma during the making of Arc Of Infinity. This impression had deepened on two further occasions: once at a party thrown by Sarah Sutton, who had played Nyssa, and later when Nathan-Turner approached Gerald Flood to appear in 1983's The King's Demons following a performance of the play Relatively Speaking, in which Baker also happened to be appearing. Unlike Baker himself, Nathan-Turner did not view his prior involvement as Maxil as an impediment to becoming the Sixth Doctor.

Nathan-Turner invited Baker to a meeting on June 10th, where the actor was astonished to find himself being offered the starring role in Doctor Who. The two men continued to discuss the proposition over the next few days and, on one occasion, ran into Davison at a hotel bar. During rehearsals for Warriors Of The Deep the next day, Nathan-Turner confirmed to Davison what his leading man had already guessed: Baker would be his successor on Doctor Who. Davison agreed to keep the information to himself until the BBC was ready to make a formal announcement. Less keen on the choice of Baker as the programme's new lead actor was Saward, who was now finding himself at odds with his producer more and more frequently.

Recording for Warriors Of The Deep got under way on June 23rd and 24th at BBC Television Centre Studio 6 in White City, London. Three months had elapsed since Davison, Fielding and Strickson had taped The Five Doctors. The first day of the block was devoted to scenes in the TARDIS, the PS Unit and Airlock Five. Problems with the costumes for both monster species quickly became evident. Ventilation was poor for the actors within, exacerbated by the heat wave afflicting England at the time. Furthermore, the Silurian outfits had a tendency to buckle around the neck, which resulted in the mask being improperly fitted to the body on some occasions. The Sea Devils' large webbed feet hindered mobility, and their helmets were too heavy, which caused the head tilt to one side and obscure the actor's vision.

John Nathan-Turner was informed that changes to the production schedule had left insufficient time to finish the Myrka

However, all of this paled in comparison to the difficulties Roberts faced with the Myrka. The monster had been designed in the manner of a pantomime horse: it required two operators, one positioned at either end of the costume. Roberts had sought out John Asquith and William Perrie to play the Myrka because they routinely operated Dobbin, a traditional pantomime horse, for the BBC children's series Rentaghost. The costume was supposed to be ready on June 23rd, to give Asquith and Perrie an opportunity to rehearse with it before they went in front of the cameras the next afternoon. That day, however, the BBC Visual Effects Department informed Nathan-Turner that the changes to the production schedule for Warriors Of The Deep had left insufficient time to finish the Myrka. Scenes involving the monster would have to be postponed to the second studio block.

Nathan-Turner informed Visual Effects that this was unacceptable: given the constraints under which Roberts was already operating, it was not possible to alter his plans to such an extent. He demanded that the Myrka be made available for recording the next day, no matter what condition it was in. Effects assistant Stuart Murdoch worked tirelessly to get the Myrka ready but, by the time the cameras began rolling on June 24th, the paint was still drying. Indeed, Fielding's outfit soon became stained through contact with the Myrka, causing a delay while it was replaced. Furthermore, Asquith and Perrie found the costume very difficult to manipulate; consequently, the Myrka was not nearly as mobile as anticipated. As a result, many of the scenes involving the monster came across as having been very awkwardly staged. This included Solow's exaggerated karate lunge, an element which Pitt had suggested based upon her own knowledge of the martial arts. Despite all the rancour, an unhappy Roberts persevered and completed the scenes in the detention area, Airlock One, and the corridor where the Myrka expired.

The difficult first studio block behind them, cast and crew travelled to the Royal Engineers Diving Establishment, at Marchwood in Hampshire, on June 28th. This facility offered a decompression chamber, as well as a water tank with a glass wall which would facilitate shots of the Doctor submerged in the hydro tank. Roberts had arranged for the tank to be made ready with warm water prior to the team's arrival, but was disappointed to discover that this had not been done. It had to be hastily filled with cold water, making the shoot a very unpleasant experience for Davison. The rest of the material originally planned for Ealing was then completed on June 29th and 30th at Shepperton Studios in Shepperton, Surrey. This included all of the material in the cooling chamber, the Silurian cruiser and the Sea Devil base.

Meanwhile, Nathan-Turner and Saward had continued to lay the groundwork for the next era of Doctor Who. They were aware that a common criticism of recent seasons had been the overcrowding of the TARDIS, and so they intended that the new Doctor would travel with just a single companion. This would be botany student Perpugilliam “Peri” Brown, who was created by Nathan-Turner and Saward in February, and would be introduced in Davison's penultimate story. Peri would be the Doctor's first American companion; although Nathan-Turner would later insist that this was an effort to diversify the regular cast, many suspected that it was intended to appeal to the burgeoning Doctor Who following in the United States. Nathan-Turner had found the name Perpugilliam in a book, and deemed it to be the kind of old English name favoured by certain upper-class American parents during the Sixties. Saward wanted Peri to be more relatable than recent companions, and he hoped that her rapport with the Doctor would evoke memories of the Third Doctor and Jo Grant from the early Seventies.

Following weeks of auditions across May and June, Nicola Bryant won the role of Peri; Doctor Who would be her first professional acting job. Unlike Peri, Bryant was British, but she was married to an American, and held joint American citizenship as a result. Nathan-Turner encouraged the actress to downplay both her marital status and her true nationality. Bryant was introduced to the press on July 5th, and was contracted for twelve episodes -- the final two serials of Season Twenty-One and the opening story of Season Twenty-Two -- on August 9th. The BBC held an option for the entirety of the 1985 and 1986 seasons.

Time grew so tight that Pennant Roberts had to use a shot of Janet Fielding which she thought was a camera rehearsal

While Bryant was being introduced to the public, four days of model filming for Warriors Of The Deep were taking place from July 4th to 7th at the BBC Visual Effects Workshop in Acton, London. Warriors Of The Deep then returned to TC6 for its second studio block; this was the only part of the schedule unaffected by the election, and so it still took place from July 13th to 15th. Material in the computer bank bay was taped on the first day, alongside some of the scenes on the bridge; these were completed on the second day, together with sequences in the chemical store. The final day involved work on the sets for the bridge entrance, the seabase corridors, and the storage area where the TARDIS materialised. Time grew so tight that Roberts found himself having to compromise on quality. In one instance, he had no choice but to use a shot of Fielding and Tara Ward (as Preston) which the actresses had thought was merely a camera rehearsal. Their visibly half-hearted performances would have to be retained in the serial's final edit. Roberts also had to abandon effects shots of the Sea Devils moving across the sea floor.

For Season Twenty-One, Doctor Who was again broadcast two times per week. After airing on Mondays and Tuesdays in 1982, and on Tuesdays and Wednesdays in 1983, Doctor Who was now scheduled at 6.40pm on Thursdays and Fridays. The Thursday broadcasts were sandwiched between the news and Tomorrow's World, occupying the slot previously filled by Angels. Doctor Who also followed the news on Fridays but, on these evenings, it led into Match Of The Day Live. This timeslot had previously belonged to the combination of Friday Sportstime and Show Business.

By the time Season Twenty-One began with Warriors Of The Deep Episode One on January 5th, 1984, it was already decided that this would be the final year for the twice-weekly schedule. Since the BBC now rarely aired half-hour dramas, it was thought that a move to a weekly forty-five-minute timeslot might freshen the appeal of Doctor Who for its twenty-second season.

• Doctor Who Magazine #199, 12th May 1993, “Archive: Warriors Of The Deep” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #1, 2001, “Hide And Seek” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #38, 2015, “Story 130: Warriors Of The Deep”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Handbook: The Fifth Doctor by David J Howe and Stephen James Walker (1995), Virgin Publishing.
• In·Vision #71, April 1997, “Production” edited by Anthony Brown, Cybermark Services.