The Doctor and Jo visit the Master, held in captivity on a small Island. The governor, Colonel Trenchard, tells them that ships have been mysteriously disappearing. They investigate and the Doctor is attacked by an underwater Silurian, a man like lizard called a Sea Devil. The Doctor discovers that the Master, assisted by a misguided Trenchard, is stealing electrical equipment from a naval base to build a machine that will control the Sea Devils - an enable him to conquer the world. The Doctor enters the Sea Devils' base and tries to encourage  peace. But his efforts are frustrated by a depth charge attack ordered by a ruthless politician, Walker. The Doctor persuades Walker to allow him a final attempt at negotiating peace but in the meantime the Sea Devils capture the naval base. The evil Master then forces the Doctor to help build his machine, which will revive Sea Devil colonies all over the world. The Sea Devils want to kill the Master, now useless to them, but the Doctor sabotages the machine and escapes with the Master. The Sea Devils are blown up.
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The Doctor:  Jon Pertwee
Jo Grant: Katy Manning

Guest Appearances:
The Master: Roger Delgado
Trenchard: Clive Morton
Robbins: Royston Tickner
Hart: Edwin Richfield
Bowman: Alec Wallis
Radio Operator: Neil Seiler
Barclay: Terry Walsh
Wilson: Brian Justice
Jane Blythe: June Murphy
Hickman: Hugh Futcher
Clark: Declan Mulholland
Sea Devils: Pat Gorman, Brian Nolan, Steven Ismay, Frank Seton, Jeff Witherick
Smedley: Eric Mason
Ridgway: Donald Sumpter
Drew: Stanley McGeagh
Mitchell: David Griffin
Lovell: Christopher Wray
Summers: Colin Bell
Watts: Brian Vaughan
Walker: Martin Boddey
Rear Admiral: Norman Atkyns
Girton: Rex Roland
Myers: John Caesar
Chief Sea Devil: Peter Forbes-Robertson
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Producer: Barry Letts.
Script Editor: Terrence Dicks.
Writer: Malcolm Hulke.
Director: Michael Briant.
Designer: Tony Snoaden. 
Costume: Maggie Fletcher.
Make up: Sylvia James.
Visual Effects: Peter Day.
Music : Malcolm Clarke.
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Novelised as "Doctor Who And The Sea Devils"  by Malcolm Hulke  (0 426 11308 X) first published by Universal Tandem in 1974 with cover by Chris Achilleos.  New edition by W.H. Allen (now Virgin Publishing Ltd.) in 1981 with cover by John Geary.  Target library number 54.

a
Released as "The Sea Devils" in the UK, September 1995 and in Australia & New Zealand (BBC catalogue #5667). Released in US & Canada 1997 (CBS/FOX catalogue #8495, reclassified as WHV #E1378). Cover art by Colin Howard. Two-tape set release in UK, US & Canada, one tape in Australia & New Zealand.

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This story introduced the aquatic cousins of the Silurians, the Sea Devils.
The British Royal Navy, waived royalty fees on the use of stock footage and clips showing ships in action, relying on the publicity generated by the show and on screen credits.

All six episodes exist in color as follows: episodes 1, 2 and 3 as PAL conversions of NTSC 2" color videotape recovered from Canada in 1983; episodes 4 and 6 on PAL 2" color videotape.

Studio recording in TC4. (4-5 October 1971)
Studio recording in TC8. (18-19 October 1971)
Fraser Gunnery Range, HMS St George, Eastney, Southsea, Portsmouth, Hampshire.(21-22, 25 October 1971)
HMS Reclaim, Portsmouth Harbour, Hampshire. (26 October 1971)
No-Man's Land Fort, Sandown, Isle of Wight. (26 October 1971)
Red Cliff, Sandown, Isle of Wight. (27 October 1971)
Whitecliff Bay, near Bembridge, Isle of Wight. (27 October 1971)
Bembridge Sailing Club, Bembridge, Isle of Wight.
...• Bembridge Harbour. (28 October 1971)
Norris Castle, East Cowes, Isle of Wight. (29 October 1971)
Priory Bay, Seaview, Isle of Wight. (28 October 1971)

Episode 1 - When the Doctor looks through a pair of binoculars at the castle in episode 1, the close-up of the castle is surrounded by a circular mask to indicate we are seeing it through the binoculars. But in episode 5 when the Admiral commanding the naval task force looks through his binoculars at the Sea Devil bodies floating in the water, we see them surrounded by a sideways-figure-of-8 mask instead.
Episode 1 - When Jo and the Doctor are climbing the ladder to get into the sea-fort, it's painfully obvious that it's not Katy Manning, but a cross-dressing stuntman wearing a Jo Grant wig.
Episode 2 - When the Doctor and Jo are carrying the fat guy out, for a split-second before the Sea Devil appears you can see shadows and light moving around in the background as if there's someone there.
Episode 2 - While in the sea-fort, the Doctor rigs up the radio to transmit messages, but somehow still gets it to receive messages too. Handy, that.
Episode 2 - The radio that the Doctor built has a fatal flaw – if you look closely at the device (this is easiest to see right before it explodes), you can see that there's a 9-volt battery clip sticking out of it. But there isn't a battery in the socket, so the whole device shouldn't be able to work at all because there isn't a complete circuit.
Episode 2 - The helicopter sent to rescue the Doctor and Jo acquires a new paint job in mid-flight.
Episode 2, 3 - The clock on the wall in the Master's quarters causes a couple of continuity problems. When the Master pulls a gun on the Doctor and the Doctor quickly pulls the door shut, the clock shows ten minutes to three. In the next episode, after the swordfight is over and the Doctor is being led out by the guards, the clock now shows approximately eight minutes after twelve!
Episode 3 - A more subtle clock problem: when the Doctor is brought into the Master's quarters and handcuffed to the chair, the clock shows five minutes to one. Less than two minutes later (still in the same scene, with no cutaways), as the Master tells the Doctor he intends to contact "our reptile friends", the clock can be seen showing exactly one o'clock.
Episode 3 - Jo leaves her handbag behind in the prison while rescuing the Doctor, but suddenly reacquires it later without any explanation.
Episode 3 - When the Master arrives at the lifebelt post, the rope is surprisingly slack, considering that the Doctor is using it to climb down the cliff.
Episode 4 - At the naval base, the search for the missing sub is called off because of night approaching. Then the sub is picked up on radar approaching the castle. We see that it's getting dark when the Sea Devils arrive at the castle and rescue the Master. Then, just a few scenes later, Captain Hart and the others are still watching the radar screen as the sub is disappearing again, and Hart says they can resume searching in that area as "it'll be light soon". And indeed, a few minutes later they arrive at the castle, and it's daylight. Where did the night vanish to?
Episode 4, 5 - When the diving bell is raised, you can see drops of water on the camera lens which change position radically between shots, betraying the fact that the finished scene was edited down from a much longer sequence.
Episode 6 - When the Doctor turns on the sonar device (causing the Sea Devils to writhe around in pain), the Master just stands there for over a minute watching a Sea Devil throwing a fit in front of him – long enough for Jo and Captain Hart to escape to the hovercraft – before saying "What are you doing?!" and turning it off!