The Doctor and Jo visit the Master, now held in captivity on a small island prison, after being captured by UNIT (at the conclusion of the previous series). The Master is being held indefinitely and is the only prisoner. He is watched by CCTV and the island is patrolled by armed guards – trained to resist the Master's hypnotic powers – and even protected by minefields. He claims to have reformed – but refuses to reveal the location of his TARDIS.
As they depart, the old-school patriotic governor, Colonel Trenchard, tells them that some ships have been mysteriously disappearing. The Doctor cannot resist investigating and he and Jo are soon attacked, while examining a Sea Fort, by an underwater Silurian. This man-sized bipedal lizard is called a 'Sea Devil' by a crew member who's been driven half mad. They escape to the nearby naval base, HMS Seaspite run by the efficient Captain John Hart; despite the Doctor's eccentirc behaviour (such as claiming to have known Nelson personally), an alliance forms. Hart is in charge of the adaptation of the sea fort for use as a SONAR testing station. The Doctor meanwhile discovers that the Master, assisted by a misguided Trenchard, is stealing electrical equipment from the naval base to build a machine that will control the Sea Devils. The evil Time Lord intends to use the reptiles as an army to enable him to conquer the planet, and he begins by using the machine to summon some of them from the sea – forcing the Doctor to employ his sonic screwdriver to repel them by exploding mines on the beach.
Soon, however, a battle for the prison ensues, during which Trenchard, who had believed he was aiding his country against enemy agents, is killed. The Doctor and the Master duel with swords, but eventually he and Jo are forced to flee to HMS Seaspite, where Hart tells them a naval submarine has disappeared. The Doctor investigates in a submarine bell and is seized by the Sea Devils, who take him to their leader. The Doctor enters the Sea Devil's base and tries to encourage peaceful negotiation, recalling how he had failed to broker an agreement between mankind and the Silurians. The Master, too, ventures to the Sea Devil base, intent on provoking war, but matters are left unresolved when the base is attacked by depth charges.
This attack has been ordered by a gluttonous and short-sighted politician, Robert Walker, who has arrived at HMS Seaspite to take control of the situation and is intent on repeating UNIT’s actions on Wenley Moor: blowing up the creatures, but this time with nuclear weapons. Hart and Jo are opposed to the attack but at least it allows the Doctor cover to flee, even if he has failed in his initial attempt to sue for peace.
The Doctor persuades Walker to allow him a final attempt at negotiation, but in the meantime the Sea Devils capture the naval base. They have been inspired to such actions by the Master, who still wishes to instigate a war. The Master then forces the Doctor to help build a machine to revive Sea Devil colonies all over the world. Returning to the Sea Devil base the Master activates the device, whereupon the Sea Devils imprison both Time Lords as they are now both equally useless to them. However, The Doctor has sabotaged the machine and he and the Master escape the base using escape equipment from the captured submarine.
As they are rescued, massive power feedback from the sabotaged machine destroys the Sea Devil colony before the military attack can begin. As usual, the Master is able to escape capture (this time by faking a heart attack and hijacking a rescue hovercraft) and flees the scene.
Doctor Jon Pertwee (Third Doctor)
Companion Katy Manning (Jo Grant)
Roger Delgado — The Master
Clive Morton — Trenchard
Declan Mulholland — Clark
Hugh Futcher — Hickman
Brian Justice — Wilson
Terry Walsh — Barclay
Stanley McGeagh — Drew
Royston Tickner — Robbins
Neil Seiler — Radio Operator
Martin Boddey — Walker
Edwin Richfield — Captain Hart
June Murphy — 3rd Officer Jane Blythe
Norman Atkyns — Rear Admiral
Donald Sumpter — Commander Ridgeway
David Griffin — Lt. Commander Mitchell
Brian Vaughan — Lt. Commander Watts
Christopher Wray — Ldg. Seaman Lovell
Alec Wallis — Ldg. Telegraphist Bowman
Eric Mason — CPO Smedley
John Caesar — CPO Myers
Colin Bell — CPO Summers
Rex Rowland — A/B Girton
Peter Forbes-Robertson — Chief Sea Devil
Pat Gorman — Sea Devil
Writer Malcolm Hulke
Director Michael E. Briant
Script editor Terrance Dicks
Producer Barry Letts
Executive producer(s) None
Production code LLL
Series Season 9
Length 6 episodes, 25 minutes each
Originally broadcast February 26–April 1, 1972