Galaxy 4 ( TV story ) Galaxy 4 was the first story of the third season of " Doctor Who " , recorded after " The Time Meddler " as part of the series ' second recording block . Its third episode , " Air Lock " , achieved the highest ratings of any episode between the final episode of " The Web Planet " and the final episode of " The Three Doctors " . Indeed , " Air Lock " is one of the few episodes from pre - BBC Wales " Doctor Who " to have ranked amongst the top fifteen shows of its initial broadcasting week . " Galaxy 4 " is the earliest non - historical story to have missing episode s . Until the recovery of " Air Lock " in 2011 , it was notable for the extreme paucity of its surviving material . Because no telesnap s were taken during broadcast , photographic evidence for this story was effectively limited to publicity shots ( with only one photograph of the Rill s ) and a six minute excerpt from " Four Hundred Dawns " . Actor Peter Purves has been outspoken in his distaste for the serial , which was originally written for Ian , Barbara and Vicki . He has frequently opined that his character , companion Steven Taylor , was not well - served because the hurried rewrite mostly transferred Barbara ' s lines to him , resulting in a " feminisation " of Steven ' s character . Synopsis . The Doctor , Vicki and Steven arrive on an arid planet where they meet the beautiful Drahvin s and the hideous Rill s . Each has crash - landed after a confrontation in space . The Rills are friendly , compassionate explorers . The Drahvins are dull - witted , cloned soldiers , terrorised by the intelligent , warlike matriarch Maaga . Both ships are damaged . The Drahvins ' craft is irreparable , but the Rills ' is almost ready to take off . Although unable to breathe the oxygen atmosphere , they employ efficient robot drones , which Vicki nicknames " Chumblies " . Despite numerous offers by the Rills to take Maaga and her crew to safety , she refuses their aid . When the planet is discovered to be on the point of disintegration , Maaga tries to force the time travellers to help her steal the Rills ' ship and kill the Rills . Instead , the Doctor allows the Rills to draw power from the TARDIS to refuel and escape , leaving the Drahvins to their fate . Plot . Four Hundred Dawns ( 1 ) . The First Doctor , Vicki and Steven Taylor arrive on an eerily silent planet . The Doctor evaluates the planet and discovers that it can sustain life , but it seems to have nothing living on it . This piques his curiosity , and he is keen to investigate this anomaly . Steven is more interested in finding somewhere to swim . Just before they exit the TARDIS the crew discover a small robot feeling its way around the ship before communicating with something else with a glowing antenna . Vicki christens the machines Chumblies . Once the Chumbley has left the TARDIS the Doctor and his companions go out and explore the planet . It has three suns and vegetation but , as yet , no sign of animals . Before they can get far , the Chumbley returns . It indicates to the Doctor and his friends that it wants them to follow it . Steven is wary . He tries to attack it . In a show of force , the machine produces a gun from its shell and sets fire to a plant . The Doctor and his companions are led away . They have not gotten far when a group of women lying in wait for the Chumbley throw a metal mesh on the machine and deactivate it . The women introduce themselves as Drahvin s , from the planet Drahva in Galaxy 4 . They say that the robots are controlled by a race called the Rill , who are not to be trusted . The Drahvins say that they will take the travellers back to their ship for safety and introduce them to their leader , Maaga . However when they try to remove the mesh they find it is attached magnetically to the Chumbley . Before they can make their escape , three more Chumblies appear and force the travellers and the Drahvins to flee . The new Chumblies remove the metal mesh from the prone Chumbley , which reactivates itself , and they head off in pursuit of the Doctor . The Doctor , his companions and the Drahvins make it to the Drahvin spaceship . There they are reprimanded by Maaga for the loss of the metal mesh . After Maaga has dismissed the Drahvins , she addresses the Doctor and his companions . She explains that the Drahvins are a powerful race of women warriors who use men only for hunting . They were exploring in this part of the galaxy when their ship was shot down by the Rill . They are desperate to get off the planet because the Rill have informed them that the planet will explode in fourteen days . Their plan is to capture the Rill ship and escape . Steven is suspicious of the Drahvin . It seems to be they who are in the wrong rather than the Rill . His position is strengthened when Maaga reveals that the Rill have offered help , but the Drahvin have refused for fear of being killed . This story is interrupted when Chumblies approach the ship . The Drahvin attack them , and the Chumblies deactivate briefly but then go on their way . Playing on the suspicions that Steven has roused in the Drahvin , the Doctor says that he is a scientist . He will learn if the planet is to explode in a fortnight and see if the Rill have been misleading the Drahvin . The Drahvin agree to this but demand one of the TARDIS crew stays with them . They insist that this is not a matter of hostages but to have fewer unaccompanied people wandering the planet . Vicki volunteers to stay and the Doctor reluctantly assents . When the Doctor and Steven arrive at the TARDIS , they find a Chumbley trying to break the TARDIS ' force barrier . Steven and the Doctor hide until the Chumbley leaves . Back at the Drahvin ship , Maaga humiliates the other Drahvins for losing the metal mesh , blaming them for ruining the whole mission . After trying to break into the TARDIS the Chumbley gives up . The Doctor and Steven enter the TARDIS . Inside , the Doctor consults his astral map and finds the planet doesn ' t have a fortnight until its destruction . It has only two days ! Trap of Steel ( 2 ) . The Doctor and Steven decide they must leave immediately , save Vicki and warn the Drahvins . However , the Chumblies return , this time with heavy explosives . It does no damage to the TARDIS . After they leave , the Doctor and Steven race back to the Drahvin ship . In the Drahvin ship Vicki is fed up with the food and is worried about her friends . Maaga refuses to let her leave . When Steven and the Doctor get back to the Drahvin ship , Steven points out it is made of a metal that the Chumblies ' ray can penetrate . The Doctor ponders why the Chumblies have not destroyed their " enemies " . Inside the ship the Doctor hides the truth from the Drahvin and again tries to convince them to befriend the Rills and work together . Maaga rejects this again . She grows suspicious of the Doctor and pulls a gun on him . Under this threat the Doctor admits they have only two days . He offers to bargain with the Rill for a peace treaty . The Drahvin agree but retain Vicki as an insurance policy . Steven offers himself as a hostage . Vicki and the Doctor leave for the Rill ship . Steven tries to create disharmony between Maaga and her minions . Maaga overhears this . She tries to convince Steven to pilot the TARDIS with the Drahvin . Steven refuses , saying that even if he wanted to , he doesn ' t know how . He is left alone and falls asleep . Trapped by a Chumbley patrol the Doctor and Vicki are forced to " observe , note , collate and then conclude " their routine . Vicki decides to test a theory by throwing a stone behind the machine . The Doctor is annoyed but she states that she thinks the Chumblies can detect only what is in front of them . They conclude that they can follow one back to the Rill ship rather than wander aimlessly . Back at the Drahvin ship Maaga orders her minions to break into the Rill spaceship and wipe them out for their ship . By this time the Doctor and Vicki are at the Rill compound . It has a drilling rig and an air purifier . The Doctor is impressed with the Rills ' handiwork . The Doctor and Vicki explore the compound cautiously . Vicki remarks that she can smell ammonia . The closer they get to the compound ' s centre , the stronger the smell . They arrive in what appears to be a repair shop for Chumblies . As they look around , a large , scaly creature appears at the window . Vicki screams . Air Lock ( 3 ) . The Doctor and Vicki try to escape the Rill centre , but they are pursued by two Chumblies . They reach a dead - end , stopped by an iron gate . The Doctor decides the best way of disarming the Chumblies is by tinkering with the apparatus that converts the air into ammonia . He assumes the Chumblies need the ammonia . Whilst he wrecks the gas converter , the Chumblies catch up with them and indicate they want Vicki to come with them . The Doctor relents , saying it will give him time to vandalise the converter . Back at the Drahvin spaceship Maaga shares her frustration with the other Drahvin warriors . She says they are useless at strategy and all they are worried about is killing . If the Doctor and Vicki succeed , she will steal the Rill ship and leave the Rill and the humans on the planet to die . She orders the Drahvins to patrol the perimeter , leaving one to guard the sleeping Steven . She does not know Steven is awake the whole time and has been listening to her . At the Rill centre , the Chumbley prods Vicki to the window where the creature appears . A voice comes from the Chumbley , telling her that the Rill control the Chumblies . At first they are hostile to Vicki . However , they learn the Drahvin have been lying to her and tell her the real story . The two spaceships met above the planet in a stand off that lasted four days . Finally , the Rill tried to leave but the Drahvin ship fired on them . The Rill ship returned fire and both ships crashed . The Rill tried to speak with the Drahvins . They found an injured Drahvin , whom they tried to help , but Maaga found them and chased them off with a gun . She killed the wounded Drahvin warrior and blamed the Rills . Vicki realises the Rill are not the enemy . If they cannot bear the planet ' s atmosphere , the Doctor ' s tinkering will kill them . She runs to stop him from destroying the gas converter . At the Drahvin ship Steven slowly raises himself silently from his bed . He sneaks up to the guard and renders her unconscious . He takes her gun and tries to leave the ship . However , as he goes Maaga comes up behind him . He finds himself facing a Chumbley . He tries to turn back but the Drahvins are pointing their guns at him . He is trapped in the airlock between the ship and the outside . Maaga gives him an ultimatum . He can surrender his gun and return to the ship , face the Chumbley or suffocate as she drains the air from the air lock . Steven is left to think as the air leaves the room . Vicki gets to the Doctor before he can break the system . Vicki explains the situation to him and they both go to the Rill . The Doctor tells them their escape is at risk . The planet will explode before they expect . Although the Rill ship is fixed they don ' t have enough fuel . The Doctor says he can help them with metal cord cable and a sort of jump lead that he has in his TARDIS . The Rills are happy to hear this , but the Chumbley outside the Drahvin spaceship tells them Steven is in danger . The Doctor and Vicki rush to his aid with two Chumblies . They are stopped by a Drahvin warrior who thinks that the Doctor has betrayed them . Vicki tries to convince the warrior that the Chumblies are under their control . The warrior tries to shoot the Chumbley , but Vicki disarms her . The Drahvin begs to be killed for her failure . The Doctor says no one will be dying on this planet any time soon . At the spaceship Steven decides he would rather face the Chumbley than return to the Drahvins . He tries the door but it cannot be opened . Steven struggles for breath . The Exploding Planet ( 4 ) . There is no time to spare . A Chumbley shoots an ammonia bomb into the Drahvin spaceship and fires its weapon at the airlock , freeing Steven . The Drahvins flee and are met by a horde of Chumblies . The Rill tell them for all these years they have been pacifist in their dealings with the Drahvin . Now that they have threatened to kill Steven , they will be killed in return . The Rill order the Drahvin back into their ship . If they are seen on the planet ' s surface again they will be killed . Inside the ship Maaga blames the Drahvin who was on patrol for their predicament . She will pay the ultimate price when they have more time . Maaga lays out her plan to destroy the Rill ship . In the Rill ship the Doctor puts the finishing touches to his plan to give a jump start to the ship ' s engine with his TARDIS . He leaves Steven to recover in the Rill centre whilst he and Vicki return to the TARDIS to link it to the Rill ship ' s engine . Alone with the Rill , Steven expresses his doubts of the benevolence of the Rills . He suspects that if the Doctor cannot fix their ship the Rill will not let them go . The Rill say they would let the Doctor go . They have no knowledge of conflict and would rather have a life saved than kill someone for no reason . Steven apologises for his scepticism . On the other side of the planet one of the Drahvin sneaks up behind the Chumbley guard and destroys it . The Rill are told the Doctor has hooked up his TARDIS . Steven is nervous . He is sure the ship will not be powered up in time . The Doctor and the Rill assure him they can complete the process before the planet explodes . A Drahvin bursts into the Rill centre . Before she can do any damage , a Chumbley paralyses her . In the aftermath Steven and Vicki look for the Doctor . They find him in the section of the Rill centre with the Rill , where they were not allowed before . The Doctor invites them in , and the Rill gives them permission to enter . At first shocked at their ugliness , the humans come to an understanding with the Rill . They bid each other farewell . Meanwhile , the Drahvins wait outside the Rill camp . They fight off an army of Chumblies to get close to the Rill ship . They find that their weapons are useless and decide to rush the ship . With the ship ready for flight , the Rill sends a Chumbley to escort the Doctor and his companions to the TARDIS . The TARDIS travellers watch as the Rill ship flies off . Nearby , the Drahvins despair as their chance of salvation disappears into orbit . Maaga catches sight of the Doctor and his companions . The Drahvin pursue them to hijack the TARDIS , but it dematerialises before the Drahvin reach their door . Seconds later the planet explodes , destroying the Drahvin soldiers and Maaga . Back on their travels , the Doctor and his companions lament that they have had precious little time for peace and quiet recently . Vicki looks at a planet below and wonders what might be going on there . On that planet , unbeknownst to the travellers , an astronaut has awakened in a dense jungle , remembering only that he " must kill " . . . Story notes . Filming locations . All episodes were filmed at BBC Television Centre TC4 , London , UK