Several ships have gone missing off the British coast, and the Doctor suspects an unnatural cause. Investigating an old sea fort at the epicentre of the disappearances, he comes face-to-face with the aquatic cousins of the Silurians, nicknamed Sea Devils. Like the Silurians, they have lain dormant for aeons in suspended animation, but have recently been awakened. But even as the Doctor starts to formulate a plan to achieve peace with the Sea Devil colony, the Master is plotting too. Having duped the governor of the prison where he is incarcerated, he now plans to help the Sea Devils destroy humanity.

In planning Doctor Who's ninth season, producer Barry Letts and script editor Terrance Dicks sought an adventure which would take place on or around the sea, a setting which the programme had only rarely utilised. To write such a story, they turned to Malcolm Hulke, who had most recently written Colony In Space for Season Eight, and who had served in the Royal Navy during World War Two. It was decided that Hulke would write a sequel of sorts to his Season Seven contribution, The Silurians. This time, however, Hulke would introduce an aquatic version of its title creatures; as such, “The Sea Silurians” was commissioned in storyline form on March 29th, 1971, and as full scripts on May 25th.

“The Sea Silurians” would also see the return of Roger Delgado as the Master. The villain had featured in all five serials aired during 1971, but Letts and Dicks had realised that the character had been overexposed by his regular appearances, and decided that it would be better to restrict him to only a couple of appearances per year. This would preserve suspense in the viewers' minds as to whether the Master was somehow involved in each adventure. “The Sea Silurians” would pick up where the Master's last story, The Daemons, had left off, and deal with the villain's incarceration.

The Ministry of Defence was eager for the Royal Navy to take part

In recent years, Doctor Who had successfully secured the involvement of the Army (for 1968's The Invasion) and the Royal Air Force (for 1971's The Mind Of Evil). Letts now decided to approach the Royal Navy for assistance with “The Sea Silurians”. Contacted in mid-June, the Ministry of Defence was eager to take part. They offered the use of men, equipment and facilities as long as the Navy was cast in a positive light.

Given the rigours of filming in maritime locales, it was decided that “The Sea Silurians” should be recorded in the autumn, before the weather became too chilly. Letts was particularly mindful of the frigid conditions endured by cast and crew while making The Claws Of Axos, the third story of Season Eight. However, “The Sea Silurians” was likewise intended to air third in Season Nine, since this would ensure that a serial set in outer space fell between each of the three based on Earth. Thus, much like The Claws Of Axos, the corresponding location shoot would fall around the onset of winter. Instead, Letts decided to take advantage of the fact that the first several adventures of the year would be completed prior to the start of Doctor Who's broadcast season. For the first time, two serials would be made out of transmission order, with “The Sea Silurians” going into production prior to The Curse Of Peladon, yet air after it.

By mid-July, Hulke had changed the story's title to The Sea Devils. He and Dicks also made an attempt to correct the naming issue which had arisen from The Silurians, namely the impossibility of the title creatures actually hailing from the Silurian period of Earth's prehistory. Instead, the Doctor would now refer to the creatures as “Eocenes”. Unfortunately, the Eocene epoch -- which began about fifty-six million years ago, and ran for just over twenty million years -- was barely more suitable, since it still occurred well in advance of the development of mankind's early ancestors.

The director assigned to The Sea Devils was Michael Briant, who had previously handled Colony In Space, and who was an experienced yachtsman. Hulke had written much of the first two episodes with an oil rig setting in mind, but Briant was unable to secure permission to film aboard one. As a result, this material was hastily reimagined as taking place on a sea fort. At the start of his location week, Briant realised that the Sea Devils were naked -- at least by implication -- and so costume designer Maggie Fletcher had to quickly dress them in blue netting.

Filming for The Sea Devils began at the Fraser Gunnery Range in Portsmouth, Hampshire on October 21st, 22nd and 25th. Situated at HMS St George, a Royal Navy shore establishment, the range was the setting for material on the beach and at HMS Seasprite. The Navy agreed to treat some of the recording as a training exercise, which facilitated the participation of a group of marines in the Episode Six battle sequences. Work on October 26th began aboard the HMS Reclaim, a diving training vessel based at Portsmouth, which served as its in-story namesake. Later the same day, the sea fort sequences were performed at the No Man's Land Fort in the Solent, just off the Isle of Wight.

The rest of the location shoot then took place on the Isle of Wight itself. On October 27th, the beach along Whitecliff Bay was the venue for the shoreline minefield. Jon Pertwee injured his ribs during recording when he dived forward and fell on the sonic screwdriver prop, which was stowed in his breast pocket. The 28th saw cast and crew relocate to the Bembridge Sailing Club in Bembridge, which provided the quayside where Robbins docked his boat. Later the same day, the jetski chase was filmed in Bembridge Harbour and at Priory Bay; the choice of vehicle came at the suggestion of Jon Pertwee, and replaced the scripted speedboats.

Location filming wrapped up on October 29th, with the day's first venue being Norris Castle in East Cowes. Posing as the Master's prison, this Norman-style fortification had been designed by James Wyatt for Lord Henry Seymour and completed in 1799. Later, cast and crew travelled to Red Cliff at Sandown, where the abseiling sequence was completed. It had originally been hoped to film this material on the 27th with marines doubling for Pertwee and Katy Manning, but fog had impeded Briant's shots. The two stars now agreed to perform the material themselves; unfortunately, Manning injured her hands when she descended the rope too quickly. The shoot's conclusion came as a particular relief to the notoriously hydrophobic Roger Delgado, who nonetheless completed all of the scenes which required him to be on or near the water.

In studio, The Sea Devils followed the usual pattern of recording in fortnightly two-day blocks on Mondays and Tuesdays, with Briant opting to tape one episode per day. All three sessions took place at BBC Television Centre Studio 8 in White City, London, although it was originally thought that Episodes Three and Four would instead be recorded in TC1. The first session, covering Episodes One and Two, occurred on November 15th and 16th. Notoriously, the opening installment showed the Master watching The Rock Collector, an episode of the children's series Clangers originally televised on April 25th. As he had previously done on Colony In Space, Michael Briant indulged himself with some voice acting for The Sea Devils, playing the disc jockey in Episode Two. Episodes Three and Four were recorded on November 29th and 30th, followed by Episodes Five and Six on December 13th and 14th. In the concluding installment, Pertwee was heard to utter the phrase “I've reversed the polarity of the neutron flow” for the first time; this would become his Doctor's catchphrase.

Malcolm Clarke's electronic score would be one of the story's signatures

To offset the expenses involved with the location filming, Briant opted to use the BBC Radiophonic Workshop to create the serial's incidental music, rather than hiring regular Doctor Who composer Dudley Simpson. John Baker was originally assigned to The Sea Devils but, after he fell ill, the duties were instead allocated to Malcolm Clarke. His highly unusual electronic score would prove to be one of the story's signatures.

Like The Curse Of Peladon before it, the early episodes of The Sea Devils were disrupted by power outages in regions across the country, stemming from ongoing industrial disputes. As a result, Episode Two on March 4th was preceded by a lengthy summary of the opening installment for those viewers who had been unable to tune in. The same night, Doctor Who gained a new lead-in, with We Want To Sing replacing Whacko!, which had come to the end of its run. A news update still acted as a bridge from the musical programme to The Sea Devils. With Episode Five on March 25th, there was also a change to the scheduled following Doctor Who, with a series of films under the High Adventure banner taking the place of It's Cliff Richard.

On March 13th, the Doctor Who office was visited by two officials from the Ministry of Defence. They were concerned that Episode Three, aired two days earlier, had included footage which featured the design for a top-secret prototype submarine. It transpired that the submarine was actually a model which visual effects designer Peter Day had adapted from a kit bought at Woolworths. With input from Michael Briant, Day had incorporated features which were intended to make the prop look more advanced, such as streamlined propellers. In so doing, he inadvertently obtained a close approximation of the Polaris submarine design actually being tested by Naval Intelligence!

• Doctor Who Magazine #192, 28th October 1992, “Archive: The Sea Devils” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Family Affair” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #18, 2018, “Story 62: The Sea Devils”, 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.