The Power of Kroll ( TV story ) The Power of Kroll was the fifth story of season 16 of " Doctor Who " and was part of the Key to Time story arc which spanned the entire season . Robert Holmes was tasked with writing a second serial for the season ( having already written " The Ribos Operation " , the season opener ) after Ted Lewis ' " Shield of Zarak " was not completed . Holmes was asked to create a monster of epic proportions . This worried many crew members , due to cost restrictions . " The Power of Kroll " was to be the fourth story of the season until it was swapped with " The Androids of Tara " , originally fifth . Aware that the K9 prop would not work on swampy ground , Robert Holmes wrote out K9 from the story . However , John Leeson took the role of Dugeen after the actor originally cast , Martin Jarvis , became unavailable . ( DWM 312 ) Synopsis . The Doctor and Romana arrive on the marsh moon of Delta Magna in search of the next segment of the Key to Time . They are caught in the conflict between the native Swampies and the crew of a chemical refinery . The presence of a gun runner complicates matters ; to make things worse , the Swampies intend to awaken Kroll , the giant god that lives beneath the swamps . Plot . Part 1 . In the far future , on one of the moons of Delta Magna , sits a vast methane and protein collection and manufacturing refinery . The staff of five — the bullying leader Thawn , his technical support Fenner , Harg , Dugeen and another crewman — are in charge of the operation . A green - skinned humanoid native called Mensch , one of the Delta Magnans , serves them . These servile people once lived on the planet but were moved to the moon when the Earth Empire identified Delta Magna as commercially valuable . Sadly for the green - skinned natives , known as Swampie s , this is now also the case on the moon . Thawn grows concerned about the possibility of insurrection when he learns that the environmentalist group Sons of Earth has taken an interest in the Swampies , and the gun - runner Rohm - Dutt is rumoured to be journeying to the moon . Thawn and Fenner go outside the refinery to see if he has arrived , suspecting a mysterious spacecraft which has touched down nearby is Rohm Dutt ' s . The Doctor and Romana I have also arrived on the moon , searching for the penultimate segment of the Key to Time . Because of the swampy terrain , K9 is confined to the TARDIS — or " marooned " , as Romana puts it . However , the Tracer they are using to locate the segment is giving some very strange readings that makes their quarry difficult to pinpoint . They are ambushed by Swampies . Romana is taken away with Rohm Dutt and a Swampie patrol . The Doctor is found by Fenner and Thawn and returned with them to the refinery . There he displays a bewildering familiarity with methane catalysation , with the refined protein shot into space for delivery to Delta Magna . The Doctor is also intrigued by the position of Mensch , whom the refinery workers do not even regard as a properly sentient being . The Doctor slips away to try to find Romana , but she has got herself into trouble . The Swampies have tied her to a rock and are about to sacrifice her to Kroll , a giant squid creature that lives under the swamps , which they worship as a god . Part 2 . Kroll appears from the waters . It is a vast squid – or rather a Swampie named Skart dressed as one . The Doctor unmasks the fake monster and rescues Romana . Back at the refinery , Dugeen has detected a huge creature some two miles wide inside the swamp . The Doctor and Romana have found reference to the real Kroll in a Swampie testament . It says the enormous creature ate a previous Swampie High Priest and has made three appearances . A fourth is due soon . Rohm - Dutt has supplied the Swampies with weapons but , when they are used against Thawn , who is searching the swamps for the Doctor , they are defective and explode whenever fired , killing one of the Swampies . Seconds later , a huge tentacle erupts from the water and seizes Mensch , dragging him to his death . In the confusion , Rohm - Dutt calls out to Thawn ; his cries go unnoticed . Everyone ' s eyes are drawn to the vast , squid - like creature that rises from the swamp , towering above the horizon . Kroll is very real , very large and very hungry . Ranquin tries to mollify it with prayer , and Kroll disappears beneath the swamp ' s surface . Despite Ranquin ' s belief that Kroll has spared his believers , the monster has simply failed to notice the Swampies , and it is soon drawn to the refinery . One of its tentacles enters a pipeline and soon smashes its way inside , dragging crewmember Harg to his death . Part 3 . The gun - runner is taken as a prisoner to the Swampie camp . The Doctor and Romana arrive and introduce themselves to the tribe and its leader , Ranquin . The elder decrees all three " dry - foots " will die by the seventh ritual of Kroll : a painful and cumbersome affair involving stretching the victims on creepers and wood that will eventually snap their backs . In fear , Rohm - Dutt admits Thawn paid him to sell the faulty weapons to the Swampies to discredit the environmentalist Sons of Earth movement . All three are secured , and their punishment begins as a violent storm starts . The Doctor rescues himself , Romana and Rohm - Dutt with " vocal vibrations " which shatter a window above , allowing the rain to pour on the vines . This relaxes them enough for the trio to escape , pursued by the Ranquin and his tribe . The three remaining refinery crew members decide to redirect the upcoming orbit shot against Kroll . They note the creature is on the move towards the Swampie settlement . When it reaches there , its first victim is Rohm - Dutt . The Doctor deduce s that the creature hunts by vibration and uses this knowledge to keep Romana and himself safe from the tentacles . The enormous Kroll rises to the swamp ' s surface , dragging a Swampie to his death . . . Part 4 . Kroll destroys the Swampie settlement , killing many Swampies before submerging again . The Doctor and Romana sneak back into the refinery and are horrified ; Thawn calculates the missile strike will destroy the creature and the Swampies . Dugeen is equally appalled . When he tries to stop the missiles being fired , Thawn kills him . The Doctor , however , disconnects the firing mechanism in the rocket silo . Thawn investigates and finds the two culprits , more certain than ever that they are Sons of Earth spies too . The tables are turned when the Swampies attack the breached refinery and kill Thawn . Ranquin , who has led the assault , believes the deaths of the " dry foots " will help appease Kroll , but the creature is now above the swamp again and hurling itself against the bulk of the refinery . While Ranquin fruitlessly prays to Kroll to stop the attack , the Doctor orders all the refinery ' s equipment activated to try to confuse Kroll . It has the desired effect , and the attack stops . Ranquin refuses to let his faith in Kroll be shaken , however . Finding one of Kroll ' s tentacles , he begs his god to destroy the " dry foots " . Instead , Kroll grabs Ranquin and drags him away to his doom . The remaining Swampies realise the creature is beyond appeasement . It is not , however , beyond science . The Doctor has worked out that Kroll has been magnified to giant proportions by a segment of the Key to Time , disguised as a Swampie relic which the squid creature swallowed with an earlier High Priest . He uses the Tracer to eliminate the giant Kroll and retrieve the fifth segment of the Key . The Doctor finally saves the refinery from destruction by aborting the automatically scheduled orbit shot , which would have caused an explosion . He tells Fenner , the only survivor of the refinery crew , to use his time wisely until a rescue mission arrives from Delta Magna and to try to understand the Swampie culture . The Doctor and Romana head back to the TARDIS with another segment of the Key in their possession . Home video releases . DVD releases . Contents ( 2007 / 2009 version ) : Notes :