The Sea Devils ( TV story ) The Sea Devils was the third story in the ninth season of " Doctor Who " . It marked the first appearance of the Sea Devil s and the reappearance of the Master , now plotting to escape confinement from a maximum security prison . Barry Letts and Terrance Dicks had decided they wanted a sea - based story and asked Malcolm Hulke to write it . Rather than merely bringing back the Silurian s , Hulke invented an aquatic version called the Sea Silurians . Unlike their earlier counterparts , these " Sea Devils " were given clothing , designed by Maggie Fletcher . To answer the insistence by fans that the Silurian era could not have spawned man - sized life , Hulke introduced a line in which the Doctor says they should correctly be called Eocenes . However , this period was still well in advance of humanoid life . After appeared in all five stories of season 8 , Letts decided to restrict him to a few appearances each year . This story joins him where " The Dæmons " left off , in prison . The dialogue implies he and the Doctor were once friends . Letts had secured the involvement of the Royal Air Force for " The Mind of Evil " and decided to try to do the same with the Royal Navy . He found the Ministry of Defence eager to take part . Many of the extras that played the navy personnel were active duty sailors . Shortly after broadcast , the BBC was visited by officials from the MOD who believed a top - secret submarine had been used in the show . It was in fact a model , adapted to show the features they were secretly testing . Expensive location filming left " The Sea Devils " with a budgeting problem . Director Michael Briant ' s solution was to not hire regular incidental music composer , Dudley Simpson , and have the score created by the BBC Radiophonic Workshop . Malcolm Clarke ' s electronic score was one of the serial ' s signatures . Its significance was attested to by its inclusion in 2013 ' s " Doctor Who at the Proms " , where it was one of five pieces of incidental music performed as representative of the show ' s original run . This story saw the only use of the catchphrase " reverse the polarity of the neutron flow " during the Pertwee era . The phrase became associated with the Third Doctor , so he used it again in " The Five Doctors " some eleven years later , although between seasons 7 and 11 Pertwee did use a shortened version of the phrase , " reverse the polarity " , several times . Like many of Hulke ' s stories , it included a moral dimension . The Sea Devils are caught between the Master , the Doctor and the humans . Ultimately , they are betrayed by all three ; like their land - based cousins , it is the humans who hurt them the most . Synopsis . The Doctor and Jo visit the Master in his high - security prison on an island off the south coast of England . The governor , Colonel Trenchard , says ships have been disappearing mysteriously at sea . The Doctor discovers that Trenchard and the Master are in league to contact the Sea Devils , a race of reptiles in hibernation in a base beneath the sea , who have been awoken by recent work on a nearby sea fort . The Master intends to use his new allies to help him conquer the world . Plot . Episode one . The Third Doctor and Jo visit , now a captive on a small island prison after being captured by UNIT . 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 disappearing mysteriously . Shortly afterwards , he visits the Master , and it becomes clear that they are in league . The Doctor cannot resist investigating , so they visit the nearby naval base , HMS " Seaspite " , run by the efficient Captain John Hart ; despite the Doctor ' s eccentric behaviour — he claims to have known Nelson personally — an alliance forms . The Doctor states that that the linear scorch pattern was caused by a concentrated beam of heat from under the sea . Hart is in charge of the adaptation of the sea fort to a sonar testing station . The Doctor and Jo make their way to the sea fort . While investigating the fort , their boat explodes . They then find the body of one of the maintenance crewman , Hickman . Jo then hears something shuffling towards them . Episode two . It is the second crewman , Clark , who is now half - mad . He is ranting about " Sea Devil s " . The Doctor and Jo try to calm him down . They find that the radio was ripped out . The Doctor decides to go get a transistor radio in order to turn it into a transmitter . On his way , he encounters a hostile reptilian Sea Devil , who fires at him . When it tries to break in , it is injured and flees . Shortly afterwards , the Doctor finishes his transmitter , and they are rescued . Captain Hart decides to find them after they apparently disappear . The Doctor and Jo attempt to get Hart to tell a higher authority . His assistant gets a call from someone ranting about Sea Devils . The Master comes to HMS " Seaspite " to steal some equipment . Colonel Trenchard follows , providing a distraction , but Jo sees the Master leaving . The Doctor and Jo confront Trenchard , and he tries to dissuade them by showing that the Master is in his cell . The Master knocks out a guard and grabs his knife . Trenchard then gets the Doctor to go and see the Master , who attempts to kill him , first with a gun and then with a sword . The Doctor and the Master engage in a sword fight . The Master then throws a dagger at the back of the Doctor ' s head . . . Episode three . The Master misses , and the Doctor is unharmed . Trenchard enters and decides to lock the Doctor in the prison and attempts to capture Jo . The Master tells him that he intends to use the reptiles as an army to conquer the planet . It is revealed that the reason Trenchard is helping the Master is because he believes they are fighting enemy agents . Meanwhile , Jo comes and frees the Doctor . The Master and Trenchard give chase , and the Master uses the machine to summon a Sea Devil from the ocean . The Doctor and Jo are trapped between the Sea Devil , the Master , and a minefield . Episode four . The Doctor is forced to employ his sonic screwdriver to repel the Sea Devils by exploding mines on the beach . While the Doctor and Jo navigate through a mine field , a Sea Devil attacks another submarine . The Doctor and Jo flee to HMS " Seaspite " , where Hart tells them another naval submarine has disappeared . Meanwhile , the Master returns to his cell to begin work on a new device . He activates it , and it calls the Sea Devils to him . They attack the prison , killing the guards . A battle for the prison rages . Trenchard , who believed he was aiding his country against enemy agents , is killed . The " Reclaim " heads out to investigate a section of seabed . The Doctor goes down in a diving bell . When they offer to pull him up , he refuses ; however , his communication system fails , and Captain Hart orders the crew to pull up the diving bell . When it reaches the ship , it is empty . . . Episode five . The Sea Devils take the Doctor to their leader . The Doctor enters the Sea Devils ' base and tries to encourage peaceful negotiation , recalling how he had failed to broker an agreement between mankind and the Silurians . The Master arrives to incite matters by trying to provoke war . Walker , the parliamentary private secretary , arrives to " solve " the sinking ship problem by dropping depth charges . Meanwhile , the Doctor has gained the upper side of the argument — the Sea Devils agree to consider diplomatic relations . The depth charges disrupt the negotiations . The Master convinces the Sea Devils to take the Doctor away and kill him . He also convinces them to attack one of the naval bases . In the confusion caused by the depth charges , the Doctor escapes , gets a Silurian gun from a fallen Sea Devil , and uses it to melt a door and rescue two of the submarine crew . They rescue the others still in the submarine . The leader of the submarine crew kills one of the Sea Devils with the gun that the Doctor picked up ; it is implied that others were killed on the upper level of the sub . They try to leave , but the Sea Devil s hold them back with a force field . They fire torpedo es to provide thrust and escape . The Doctor confronts Walker about his depth charges , which have just made the Sea Devils angry . The Sea Devils rise and attack the HMS " Seaspite " . The Doctor attempts to go to negotiate again . As they round a corner , a Sea Devil appears and raises its gun at them . . . Episode six . The Doctor fights the Sea Devil off with Venus ian karate , but another captures him . The rest of the Sea Devils capture the rest of the people at the base . Jo , Captain Hart , and Mr Walker are held in one of the offices at the base . The Master tells the Doctor that he needs help to complete his machine so he can revive the colonies all over the world . Meanwhile , Jo escapes through a ventilation shaft . She reaches the Doctor , and he tells her that he will deal with the guards ; he sends her back to retrieve Captain Hart and Mr Walker . The Doctor makes the machine emit a shrill noise , which puts the Sea Devils in agony . Captain Hart escapes with Jo , but Mr Walker retreats into the office and shuts himself back in when faced with a Sea Devil , even though the creature is incapacitated by the noise from the machine . The Master shuts off the machine , and the Doctor apologises for the " mistake " that caused the noise . The Sea Devils , the Doctor , and the Master head back to the Sea Devil base . The prisoners retake the base . A soldier arrives , and the Doctor orders him to keep watch over the Master , but the Master hypnotises him and escapes . The Doctor chases after the Master , and they reach the base , where multiple Sea Devils capture them . Meanwhile , Walker orders a nuclear strike . The Doctor sabotages the machine , and the two Time Lord s are imprisoned . The Doctor reveals that he reversed the polarity of neutron flow , which will cause a massive explosion . The Time Lords escape by using the Doctor ' s sonic screwdriver to get out of their cell ; they find diving suits and exit the base . They rise to the surface and are rescued just before the base explodes , and the Sea Devils are destroyed . The rescuers call an ambulance for the Master , who seems to be very ill ; however , when the ambulance arrives , they realise that the man is not really the Master , but someone else wearing a disguise — the Master has hypnotised him and forced him to impersonate the Master . They realise that the Master is escaping in a hovercraft . Too late to do anything to stop him , all they can do is bitterly watch their foe taunt them as he scurries away to freedom . Home video and audio releases . Editing for both the VHS and DVD releases was completed by the Doctor Who Restoration Team . DVD releases . This story was released on DVD as part of the " Beneath the Surface " collection along with " Doctor Who and the Silurians " and " Warriors of the Deep " . Digital releases . This story is available : VHS releases . This story was released on video in episodic format . Audio releases . A CD of the original television soundtrack was released in January 2008 , with linking narration by Katy Manning . It was also included in the " Monsters on Earth " box set along with " Doctor Who and the Silurians " and " Warriors of the Deep " .