The Master escapes from incarceration and allies himself with the newly-awakened Sea Devils, the aquatic cousins of the Silurians, who have also lain dormant for aeons in suspended animation. With an army of Sea Devils at the Master's command as he seeks to conquer the Earth, the Doctor soon realises that even with the backing of the British navy, there may be no stopping his arch-foe this time.

In planning Season Nine, Doctor Who producer Barry Letts and script editor Terrance Dicks decided to include one serial which would be set on or around the sea, something which had been done only rarely in the programme's history. To write such a story, the production team turned to Malcolm Hulke, who had most recently written Colony In Space for Doctor Who's eighth season. It was decided that the maritime adventure would see a return, of sorts, of the title monsters from Hulke's Season Seven contribution The Silurians. This time, however, Hulke would introduce an aquatic version of the creatures; as such, “The Sea Silurians” was commissioned in storyline form on March 29th, 1971, and as full scripts on May 25th.

The story would also see the return of Roger Delgado as the Master. The villain had featured in all five Season Eight adventures, but Letts and Dicks now realised that this had overexposed the character, and decided to restrict him to only a couple of appearances per year. “The Sea Silurians” would therefore pick up where the Master's last story, The Daemons, had left off and deal with the villain's incarceration.

During Season Eight, Letts had been successful in securing the involvement of the RAF for The Mind Of Evil, and so decided to turn to the Royal Navy for support on “The Sea Silurians”. Contacted in mid-June, the Ministry of Defence was eager to take part so long as the Navy was cast in a positive light, offering the use of men, equipment and locations.

It was realised that filming in naval locales would most comfortably be carried out before the autumn weather became too chilly. However, “The Sea Silurians” was planned to be the third story of Season Nine (ensuring that there was an adventure set in outer space in between each of the three based on Earth), suggesting a December or January location shoot. Consequently, Letts decided to take advantage of the long lead time between production and transmission afforded Doctor Who since Jon Pertwee's debut and make “The Sea Silurians” second as Serial LLL, while still maintaining its original place in the season schedule. This would therefore be the first instance of a story being made out of broadcast order in the show's history.

By mid-July, Hulke had changed the story's title to The Sea Devils. He and Dicks had also made an attempt in the scripts to correct a perceived error in The Silurians, namely the impossibility of the title monsters actually hailing from the Silurian period of Earth's prehistory (about 400 million years ago). Instead, they now had the Doctor referring to the creatures as “Eocenes”, but unfortunately the Eocene epoch (about 16 to 54 million years ago) was barely more suitable, as it still came well in advance of the development of mankind's very early ancestors.

The director assigned to The Sea Devils was Michael Briant, who had previously helmed Colony In Space. Upon arriving at Fraser Gunnery Range at HMS St George in Portsmouth, Hampshire (the setting for scenes on the beach and at HMS Seaspite) for the first two days of filming on October 21st and 22nd, Briant realised that the outfits for the Sea Devils implied that they were naked. Consequently, he had costume designer Maggie Fletcher dress the Sea Devils in blue netting.

Briant's team returned to the Fraser Gunnery Range for one more day on October 25th. On the 26th, the nearby HMS Reclaim, a diving training vessel, served as its in-story namesake while the sea fort sequences were performed at the No Man's Land Fort in the Solent, just off the Isle of Wight. The sea fort setting was a late change to the scripts: Hulke had originally written this material for an oil rig, but Briant was unable to obtain permission to film on one.

The rest of the location shoot then took place on the Isle of Wight itself. On the 27th, Whitecliff Bay and Red Cliff, Sandown were the venues for the sequences involving the shoreside minefield. Jon Pertwee injured his ribs during recording when he dove forward and fell on the sonic screwdriver prop, which was sitting in his breast pocket. October 28th saw cast and crew relocate to Bembridge Sailing Club and Bembridge Harbour in Bembridge, for material at the quayside and at sea. Additional beach shots were completed on the same day at Priory Bay, Seaview. Location filming wrapped up on the 29th at Norris Castle in East Cowes, which served as the prison. The conclusion of the exterior material came as a particular relief to Delgado. Notoriously afraid of the water, the actor had nonetheless persevered and recorded all the scenes that were asked of him.

For the studio recording, Briant followed the usual Doctor Who pattern of recording one episode per day. The first session took place on Monday, November 15th and Tuesday the 16th, in BBC Television Centre Studio 8. In episode one, the Master was shown watching The Rock Collector, an episode of Clangers originally televised on April 25th. For part two, Briant himself provided the voice of the radio DJ. The second studio block occurred on the 29th and 30th; the venue was again TC8, although it was originally thought that TC1 would be employed instead. TC8 was used once more for the final two studio days, December 13th and 14th.

To offset the expenses involved with the location filming, Briant opted not to hire regular Doctor Who incidental music composer Dudley Simpson, but instead have the score created in-house by a member of the BBC's Radiophonic Workshop. John Baker was originally assigned to The Sea Devils but after he fell ill, the duties were allocated to Malcolm Clarke, whose unusual electronic score would prove to be one of the serial's signatures.

Amusingly, on March 13th, two days after the transmission of The Sea Devils part three, the Doctor Who office was visited by two officials of the Ministry of Defence, who were concerned about footage in the episode they believed was of a top-secret prototype submarine. It transpired that the submarine was actually a model devised by visual effects man Peter Day who, in an effort to make the prop look more advanced, had accidentally included design features -- most notably sleeker propellers -- similar to those the government was actually testing.

• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.
• 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 #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.