The Krotons ( TV story ) The Krotons was the fourth story of season 6 of " Doctor Who " . It was the first story penned by future script editor Robert Holmes , whose prolific contributions to the series created or contributed to many key elements of the programme ' s mythology . Synopsis . The TARDIS arrives on the planet of the Gond s , who are ruled and taught in a form of self - perpetuating slavery by the alien Krotons — crystalline beings whose ship , the Dynatrope , crash - landed there thousands of years earlier after being damaged in a space battle . The Krotons are in suspended animation , in a crystalline slurry form , awaiting a time when they can be reconstituted by absorption of mental energy . Periodically , the two most brilliant Gond students are received into the Dynatrope , nominally to become " companions of the Krotons " , but in truth to have their mental energy drained , after which they are killed . When the Second Doctor and Zoe take the students ' test , their mental power is sufficient to reanimate the Krotons . The Doctor discovers that their life system is based on tellurium . With help from the Gond scientist Beta , he destroys them and their ship with an impure form of sulphuric acid . Plot . Episode one . On the planet of the Gonds , a ceremony is underway . The two brightest students , Abu and Vana , have been selected to become " companions " of the Gonds ' unseen masters , the Krotons . As Abu eagerly dons his selection robes and enters the Krotons ' machine , Thara desperately begs Vana not to go . The Gonds argue with him . She has been selected for the highest honour . Those companions who have gone before have never been seen nor heard from again . The Gond leader Selris , Thara ' s father , is resolute , stating that Kroton law dictates that she must . The Doctor ' s TARDIS lands in a desolate area nearby . Jamie immediately notes the strong odour of sulphur whilst the Doctor states that the planet has twin suns . Zoe suggests that they depart , but the Doctor opts to explore . From a clifftop they see the Gond city below . They encounter a metallic crystalline structure with a hatchway similar to the one in the city . Jamie noticed that the smell is stronger . The Doctor is cautious . He identifies the structure as a machine and wants to head back to the TARDIS . As they head off , the door opens , and they hide as a dazed and weary Abu emerges from the hatchway . Before their horrified eyes , he is evaporated by blasts of energy . Thara and Selris argue over Vana ' s selection . Thara questions if the Krotons even exist . Thara draws a weapon and is soon surrounded by Gonds . The Doctor , Jamie and Zoe enter the city . They are regarded with suspicion ; the Gonds have never met strangers before and the area where they landed , the Wasteland , is believed uninhabitable . Meanwhile , the stand - off between Thara and the Gonds continues . Vana wants to go through the door in order to protect Thara . He relents . The Doctor , Jamie and Zoe watch this , and the Doctor tries to stop it , thinking it some kind of sacrifice . The Gonds seize the Doctor and his colleagues for stopping their ritual . As a particularly hotheaded young man named Axus fights with Jamie , Vana rushes into the Kroton machine . Jamie wins the fight with Axus . Selris interrogates the Doctor and his colleagues . The Doctor ' s description of the death of Abu shocks the crowd , as does the revelation that they have been in the Wasteland . The Doctor , Jamie and Zoe rush back to the Wasteland to rescue Vana , followed by Thara . They all reach the door . The Doctor sets about finding stones and starts blocking the energy beams with rocks . They hide . Soon Vana appears and swoons . The Doctor swoops in with his umbrella up to save her . The gas knocks the stones free , which causes the Doctor ' s umbrella to disintegrate , but everyone is safe . Vana is near - catatonic . They all set off for Selris ' house . Back in the city , the Doctor tends to Vana through hypnosis . Selris tries to make sense of what is happening . Have they always been murder ing the selected Gonds ? The Kroton machine landed on the planet thousands of years earlier , and the ensuing war nearly annihilated the Gonds and ruined the landscape . Since then , the Krotons have never left their machine . Thara and a band of students sneak into the Learning Hall and interrogate the Curator for information about the Krotons . The Curator states he is forbidden to tell and that they only communicate through messages and , very occasionally , voice . They decide to fetch the Krotons out by smashing their teaching machines . The Doctor and his colleagues learn that the Krotons created the teaching machines to train the Gonds on the laws , science and culture that require their top students to become companions ; the Doctor notes this is a form of self - perpetuating slavery . Selris panics about what this means for his society . Beta , their chief scientist , arrives and tells Selris of Thara ' s actions . Selris worries he will not reach the Learning Hall in time ; however , the Doctor states that they can cross through the Wasteland . Selris is reluctant but soon relents . Thara and his gang vandalise the Kroton teaching machines to force them to emerge . They are stopped by a bellowing , mechanical voice ordering them to leave the hall . Thara wants to push on to bring the Krotons out . The Doctor , Selris and everyone else arrive and unarm the vandals and reason with them . Somewhere inside the machine , a check is run on the Doctor . In the Learning Hall a portal opens and a metallic , snake - like object emerges and moves towards the group . Inside the machine all screens display an image of the Doctor . The probe homes in on the Doctor who backs away . The Doctor falls and covers his face as the machine bears in . . . Episode two . The device stops when the Doctor shields his face . Zoe notes that the device must use pattern recognition . This is confirmed when the Doctor deliberately lowers his hands momentarily and it comes back to life . A student attacks the device , which vaporises him and then retracts into the Kroton machine . The Doctor notes the apparently limited logic of the device ' s programming ; it seems to think it has completed its task of destroying him . Thara wants to continue destroying the machines , but Selris stops him . The bellowing mechanical voice returns , ordering them to leave the hall . Selris orders the Gonds back . The Doctor checks up on Vana . She is getting better . The Doctor and Zoe note the large gaps in the Gonds ' knowledge . They have a great knowledge of chemistry and medicine but no knowledge of electricity . The Krotons apparently are training the Gonds for a specific purpose . They leave Jamie to care for Vana and go look round the Learning Hall . Selris says he will come with them . The Doctor , Zoe and Selris enter the Learning Hall . The Doctor investigates a closed - off chamber running under the Kroton machine , and he and Selris go to investigate . Left alone , Zoe ' s curiosity forces her to experiment with the learning machines . She becomes enthralled as she operates it . The Doctor and Selris come back , saying it is only the foundations of the machine ; however , the Doctor has noticed something is peculiar . Before he has a chance to explain , he notices Zoe on the machine and pulls her off it and berates her . Zoe has a big smile on her face and says that the Krotons are happy with her . The Doctor tries to snap her out of this brainwashed state . Selris notices that she has registered twice as high as the record for a Gond score . Vana wakes up . She begins to babble about a " burning ball " . Thara tries to calm her , but she continues to talk about " flashing lights " and her " burning mind " . She briefly recognises Thara but then goes back into her reverie . Jamie gives her some pills the Doctor gave him which send her back to sleep . Jamie leaves to tell the Doctor of the situation . The Doctor and Zoe are inspecting one of the walls in the Gond city and find it covered in some kind of crystalline substance . The Doctor states that underground he found that the machine ' s foundation had a root structure , suggesting the Kroton machine may be organic . A gong sounds , and a message from the Krotons is delivered . Zoe has been summoned to be a companion of the Krotons . The Doctor takes the test himself so as to accompany her . He does not perform as well as Zoe , scoring higher but requiring more questions . The Doctor comes away from the machine stating that he is happy he has pleased the Krotons , before realising he too is becoming brainwashed . The gong sounds again , and the Doctor has been summoned also . The door opens . The Doctor and Zoe have lost a bit of their bravery but edge their way in . As the door closes , Jamie enters and sees his two friends enter the dangerous machine . The Doctor and Zoe find the interior chamber empty , save for two chairs . They sit and are subjected to a force generator that keeps them glued to the spot . The Doctor asks Zoe to hold onto the other end of a chain which may equalise the power load and save them . The machine lights up and bears down on them . Jamie is furious with Selris for allowing the Doctor and Zoe to enter the machine . He tries to get in . The Doctor and Zoe struggle against the machine . They succeed and awake . Zoe notes that the device must convert mental power into energy . They see some bubbling tanks and realise that , whereas the Gonds ' mental energy was insufficient , their mental energy set the tanks into operation . The Doctor takes a sample and realises that it is a crystalline slurry for making life . Zoe discovers some pipes on the floor , similar to an astronaut ' s airlines . They notice something forming in the tanks . As it starts to climb out , the Doctor and Zoe escape . As they do , two Krotons , crystalline bipeds , plug themselves into the airlines and watch them on their scanners . Outside Jamie is still trying to get in . He moves off in search of a crowbar . The Krotons wonder why the Doctor and Zoe are not following the same pattern as the other Gonds . They speculate as to whether the conditioning is failing . The Doctor and Zoe make their way through an elaborate series of pipes and find a photo - electronic circuit that should open the door , but it has been disabled . The Doctor uses a piece of metal he has with him to complete the circuit manually . They door opens , but they realise that they are on the cusp of where the deadly gas vaporises the Gonds . The Krotons watch the reasoning of these Gonds in shock . Zoe decides to risk it , but they should drop down the side of the ramp and escape that way . They do so . The Krotons decide to keep monitoring the Doctor and Zoe whilst also ordering the Gonds to bring them back . They speculate that they are not Gonds . They look in the Learning Hall and see Jamie attempting to enter . He is also not a Gond , but another Gond , Selris , is watching on . They speculate the Gonds are working in alliance with a new race . They decide to take Jamie . The door opens . Jamie , armed with the crowbar , enters . He is immediately gripped by a Kroton . They test his mind . The machine starts , and Jamie is paralysed . He tries to resist . The Krotons realise he has a primitive brain . They state that the machine will destroy him if he does . Jamie slumps to the ground . . . Episode three . One of the Krotons determines that Jamie may be able to provide information about the Doctor and Zoe and turns off the mind probe . The Doctor decides to head back to the TARDIS . Zoe questions the safety of Jamie , but the Doctor perseveres . The Krotons question Jamie as to where he and his friends come from . They ask about the TARDIS , and Jamie is forced to answer truthfully by the mind probe . They watch as the Doctor and Zoe enter the TARDIS . Jamie questions if they are heading off without him . The Krotons deliberate whether to destroy the TARDIS . A group of Gonds , led by security chief Eelek , approaches scientist Beta about how to attack the Krotons now that the Doctor and Zoe are presumed dead . Beta is reluctant , not out of fear as Eelek infers , but out of a lack of knowledge . The Krotons have taught the Gonds only what they want for them to know . Beta bemoans the inability of the Gonds to think for themselves and thinks they are just as unprepared and primitive as their ancestors . Eelek is impatient to act against the Krotons . He reveals there has been a vote amongst the counsel of the Gonds and that he has been announced the new leader . They decide not to fire . They question Jamie about the operating principle and the transference interval of the TARDIS , but Jamie cannot answer any of those questions . A Kroton is sent out to investigate . Back in the Gond city , Thara tends Vana . Selris comes and tells of the fact that the Doctor , Zoe and Jamie have entered the machine and that he is going to call a counsel meeting to discuss what to do . Thara is resentful of Selris ' s apparent inaction and tells him of Eelek ' s power grab . The remaining Kroton tells Jamie he is to be killed . Thara tells Selris that the Gonds are fed up with talking and want war . Although Selris considers the Krotons enemies , he is more cautious about how to fight back than Eelek . He doubts Eelek ' s intentions and sees his ambition as more of a power grab . Jamie questions what the Doctor and Zoe are needed for . The Kroton explains that high ability brains are needed for transferring power into the Dynatrope , the Kroton ' s name for their machine . At the council meeting , Beta openly argues with Eelek . The Kroton tells Jamie that there is no way of destroying a Kroton . They can be rendered into molecular form but will always survive . Jamie is edging towards a weapon that he saw the other Kroton arm himself with . Meanwhile the Doctor and Zoe have analysed the substance that they found in the tank and have discovered that it is tellurium . They have also discovered that the smell in the air is hydrogen telluride . The Doctor wants to extract some of the sulphurous rock that is producing this gas . Zoe fears they are being watched . They are , by a Kroton . Jamie watches on the screen and implores the Doctor and Zoe to get back in the TARDIS . Zoe sees the Kroton , and it orders them to return to the Dynatope . Jamie attacks the other Kroton ; it is disoriented . The Kroton is unable to see in the light and has lost contact with the other Kroton ; the Doctor and Zoe are able to escape . The other Kroton must ignore Jamie to provide directional data and instructs the Kroton to destroy the TARDIS . The Kroton fires a dispersion jet . Zoe is horrified when the mist clears and the TARDIS is gone , but soon after the Kroton returns to the Dynatrope , the TARDIS re - materialises . The Doctor turned on the HADS ( Hostile Action Displacement System ) before leaving , and they return to the Gond city . Eelek and his men prepare to attack the Dynatrope directly . Selris enters and demands Eelek stop . Eelek ignores him and leaves with his supporters . Selris has his own plan , which he shares with Beta , to attack the support pillars under the Dynatrope ( accessible via the closed chamber in the Learning Hall ) . He hopes the attack will force the Krotons to emerge so they can be attacked directly . The Kroton tells Jamie that , without high ability brain power , the Dynatrope will expire in three hours . Jamie escapes . The Doctor and Zoe return to find Thara with the recovered Vana , preparing to leave the evacuated city . The Doctor is alarmed when Thara informs him of Selris ' s plan . He hurries off to the learning hall . Selris is putting his plan into operation . The Doctor stops off at Beta ' s lab to leave the sulphur and instructions . His alarm heightens when he realises Jamie may still be in the Dynatrope . The Doctor and Zoe rush off , but Thara and Vana stay with Beta . The Krotons notice that their Dynatrope is unbalancing . Selris watches as cracks begin to form in the roof of the base unit of the Dynatrope . The Doctor enters the underchamber , yelling for them to stop . Their efforts cause a massive rockfall . Some of the rubble falls on the Doctor . . . Episode four . Thara and Vana head down to the foundations of the Dynatrope . They feel the rumbling . Thara rushes down . Thara joins Zoe and tries to save her , but she refuses to leave without the Doctor . Thara sets off to look for him . Vana finds Selris and tells him that Thara is looking for the Doctor and Zoe . Selris worries for the Doctor . The Doctor is shaken but otherwise unhurt , though several Gonds are injured . Thara is added to the number of injured when a further rock fall lands on his leg . The Doctor and Zoe pull the rock off . Jamie staggers around the tremulous Dynatrope looking for an exit . The Krotons are attempting to readjust their Dynatrope , eventually rendering it stable . They realise that the Gonds have attacked them in association with the Doctor and Zoe . They send out to get them back . Thara has fractured his leg . The Doctor wants to abandon the Learning Hall . Selris joins them and regrets that his actions have not effected the machine at all . The Doctor disagrees and shows that it is leaking and probably not running at full power . Vana provides the Doctor with a small sample of Beta ' s sulphuric acid . The Doctor is horrified to learn that Jamie is inside the Dynatrope and rushes off . In an effort to escape , Jamie tries to pull the door open but then finds the metal the Doctor used . Eelek and Selris argue over leadership . Eelek blocks all of Selris ' ideas and is preparing for war . He attempts to arrest Selris , but all are distracted when a Kroton emerges from the Dynatrope , demanding the two high - brains ( the Doctor and Zoe ) be returned . Their minds are needed to produce energy to repair the drive mechanism , and the Dynatrope is quickly running out of energy . The Kroton kills a Gond and says they have fifteen minutes . Eelek promises that they will comply . Through trial and error , Jamie manages to open the door a crack Eelek and Selris argue . The opportunistic Eelek realises that if they are captured , the Krotons will leave , though Selris suspects that the Krotons will destroy the Gonds regardless . The Doctor and Zoe discover Jamie stuck in the door and pull him out , narrowly avoiding the gas . The Doctor checks Jamie is OK and sends him off to Beta to prepare more acid before rushing off . Vana and Selris decide that what Eelek is doing is unfair and that the Doctor and Zoe should be warned . Jamie aids Beta in the construction of the acid — although neither really know what they are doing . Vana and Selris come up with a plan . Selris is to distract the guards , whilst Vana escapes and warns their friends . This plan is foiled , and as Vana is brought back , the bottle of acid falls from her . Selris picks it up . The Dynatrope will power down in 27 minutes . Eelek and his men capture the Doctor and Zoe as they enter the city . The Krotons watch this . They now have 22 minutes . As they are forced into the machine , the Doctor tries to reclaim the acid from Van , but she no longer has it . The Krotons decide not to destroy the Gonds yet as it will use up power . Vana asks Selris if he has seen the acid . Selris realises what has happened and jumps into the Dynatrope as the door closes . The Doctor and Zoe come face to face with the Krotons . As Selris appears and gives the acid to the Doctor , he is destroyed by a Kroton . The Krotons begin their take off . They have twelve minutes . Zoe suggests they should stall for time , but the Doctor points out that if the machine powers down it will explode and destroy the planet . The Doctor manages to get the Krotons to explain that they need four brains working in tandem to power the ship . There used to be four Krotons , but two got destroyed in a space battle . This forced them to land on the planet of the Gonds and go into perpetual stability whilst they train the Gonds to assist them . During this story the Doctor surreptitiously hands the acid over to Zoe . There are now nine minutes . Jamie and Beta make their way down into the basement of the Learning Hall carrying a huge cauldron of acid . Eelek tells him that the Doctor and Zoe have reentered the machine . Jamie confronts Eelek for forcing them into it . The Gonds are evacuating the city as it will be destroyed when the Dynatrope leaves . Jamie , Beta , Vana and Thara stay . The Krotons order the Doctor and Zoe to put headsets on as there are only six minutes left . As the Doctor distracts the Krotons , Zoe pours the acid into the tank into which their airlines are connected . The Doctor and Zoe stall for time but are eventually left with no option other than to put the headsets on . Just at the last minute , the Krotons begin to feel the effect of the acid . They fire their weapons , forcing the Doctor and Zoe to duck . When they emerge , they see the Krotons are dissolving . The Dynatrope also seems to be disintegrating . Jamie , Beta and some more Gonds are pouring acid into the Dynatrope . They are joined by the Doctor and Zoe . They watch as the machine melts . The Doctor , Zoe and Jamie sneak away . Thara vows he will deal with Eelek , and Beta says he will become the true scientist he wishes he could be . He is only disappointed when he realises the Doctor isn ' t around to aid him . In the Wasteland , the TARDIS dematerialises . Home video and audio releases . Digital releases . This story is available :