The Armageddon Factor ( TV story ) The Armageddon Factor was the sixth and final story of season 16 . It concluded the season - long Key to Time story arc . Mary Tamm makes her final appearance as Romana I while her replacement , Lalla Ward , appears in the story in a different role . The Black Guardian appears for the first time and isn ' t seen again until " Mawdryn Undead " . Synopsis . The final segment of the Key to Time is at the heart of a devastating war between neighbouring planets Atrios and Zeos . The Fourth Doctor discovers that a sinister entity is manipulating events and the cost of obtaining the final segment may be more personal than he imagined . Plot . Part one . The sixth and final segment of the Key to Time is on the planet Atrios . Twin planets Atrios and Zeos are locked in a long - running war . The young Princess Astra , nominal leader of Atrios , is appalled at the devastation , but the Marshal , in charge of the war , actually possesses the power . The Marshal secretly confesses to his aide - de - camp Shapp that they are losing . He is desperate for the edge that will bring victory . On instruction from an unseen entity , he leads Astra on a fool ' s errand into a trap , where she is abducted and transmat ted away . The Doctor and Romana I land on Atrios , and the Doctor ' s TARDIS is soon buried in rubble from a Zeon aerial bombardment . The Marshal finds them and believes them to be spies , along with Astra ' s lover , Merak . The Doctor tricks the Marshal to calling for K9 , who knocks out the lights to let them escape . Going to the place they landed , they find the TARDIS gone . . . Part two . Merak has followed them , and they find Astra gone . K9 , meanwhile , is lured away to be recycled . The Marshal is contacted by an unseen force , who tells him to treat the Doctor and Romana as guests . The Marshal implores the Doctor to assist Atrios . The Doctor proposes a shield that will stop the Zeons from attacking , but the Marshal insists he create a weapon for total victory . Romana notices that the Marshal is acting under an outside influence when he accidentally shows a small device on his neck after flipping out when the Doctor runs to save K9 from the furnace . Afterwards , the Doctor asks the Marshal to send him to Zeos . The Marshal refuses . The Doctor reveals to Romana that something is probably blocking Zeos . Astra appears on a television and states that the Zeons will destroy Atrios if the Marshal does not surrender . The Doctor is trapped by two people in masks in a transmat . Part three . The unseen entity tells Marshal that the war will stop , while the entity traps him in his domain . It calls itself the Shadow , who reveals that he has the TARDIS . The Shadow asks the Doctor to open his TARDIS and bring out the other five segements and then leaves . The Doctor decides to search for the sixth segment . Merak , meanwhile , has transmatted to the Zeos . Astra is on the ship in chains , being asked by the Shadow where the sixth segment is . Romana and K9 transmat to Zeos , following Merak . Shapp finds himself with the Doctor after being transmatted . K9 finds the commandant of the Zeon forces , which is actually a supercomputer named Mentalis . The Marshal , meanwhile , has decided to attack Zeos . They find that Mentalis has been instructed to conceal any information on Astra . The Marshal , in his spaceship , prepares to launch the missiles to destroy Zeos . . . Part four . They learn that if it is destroyed , it will destroy Zeos and Atrios — a concept known as the Armageddon Factor . The computer can ' t counterattack because it ' s been told that the war is over . They discover the Marshal , in a last - ditch effort , is piloting the last Atrian warship to destroy the Zeon capital with a nuclear missile . The attack will set the mutual destruction in motion . Astra is hypnotised by the Shadow , and a projection of her lures Merak , and he begins falling . The Doctor is , meanwhile , trying to reinstruct Mentalis but accidentally triggers the computer into waiting for an attack to blow itself up immediately . The computer becomes mindless by destroying its own control centre . Desperate to stop the Marshal ' s attack , the Doctor uses the five segments of the Key to Time , plus an artificial sixth segment made from chronodyne , to generate a temporary time loop around the Marshal ' s ship . Meanwhile , K9 is hypnotised by a small device which takes the guise of a distress call . K9 is then transported away , where he is reprogrammed . The TARDIS heads to the third planet . On the third planet , the Shadow laughs that the Key to Time is his . . . Part five . The Doctor , Astra and Romana are on the third planet , and each one is separated from the others . The Doctor finds another renegade Time Lord , Drax , in the corridors of the third planet . Drax was employed with threat of death , and he was forced to build Mentalis . The Doctor persuades Drax to assist him , and they remove the control device from K9 . Drax repairs his dimensional circuit . They uncover the ultimate truth : a third party , known as the Shadow , is manipulating the entire war from his enormous vessel midway between the two planets . An agent of the Black Guardian , the Shadow has been watching the final segment of the Key ( Princess Astra herself ) , setting the war in motion and simply waiting while the Doctor risked life and limb to find the first five segments . The Shadow forces the Doctor to get the Key . Arriving at the TARDIS , the Doctor tries to distract the Mute escorting him by saying the Shadow will kill him and his fellows . Drax then enters with the dimensional circuit and shrinks the Doctor down . Part six . Drax also shrinks himself , and they decide to have one of them provide a distraction . Romana , meanwhile , realises that Princess Astra is the sixth segment to the Key to Time . Merak transports himself to the third planet . Drax and the Doctor return to normal size and take the Key to Time from the Shadow . Merak stays behind to look for Astra , who has turned into the sixth segment . When the artificial time loop expires , the Doctor and Drax disarm Mentalis . When the Marshal fires , the war rockets are deflected by a force field , destroying the Shadow and his ship instead . Hearing his minion ' s dying words , the Black Guardian disguises himself as the White Guardian , telling the dying Shadow that he will trick the Doctor into giving him the Key to Time himself . Astra converts herself into the final segment , completing the Key to Time , and giving the Doctor , for the moment , absolute power over the entire universe . The White Guardian appears on the TARDIS monitor . He demands that the Doctor hand the Key over to him . When he rather callously dismisses Astra ' s sacrifice , the Doctor realises this is actually the Black Guardian in disguise . The Doctor orders the Key segments to disperse across the universe , which also allows Astra to reunite with Merak . The Black Guardian is furious and threatens to destroy the Doctor . However , the Doctor has installed a randomiser on the TARDIS console , ensuring that neither he nor the Black Guardian knows where he ' ll end up next . Story notes . Outtakes and gag reel footage . Several clips of scene performances not intended for broadcast have been circulated from this serial , including two sequences videotaped during rehearsal ( Mary Tamm is seen wearing glasses and hair - curlers ) . In one scene , Tamm and Baker jokingly pretend to move in for a kiss after delivering a line , and in another widely circulated clip , the Doctor replies to a negative comment from K9 , " You never f * * * * * * know the answer when it ' s important ! " Off screen crewmembers laughed at both of these outtakes . It was also during production of " The Armageddon Factor " that Baker , Tamm and John Leeson filmed a brief one - minute gag scene dubbed " Doug Who ? " for the BBC staff Christmas party . The scene begins with the Doctor and Romana sitting on the floor by the TARDIS console , apparently kissing off screen , and then acting tipsy as they share a bottle of vodka with K9 , who is asked to sing a few bars of " We Wish You a Merry Christmas " . The Doctor then asks K9 what he wants for Christmas ; K9 replies and then asks the Doctor for what his desire is , to which the Doctor looks into the camera and then leers at Romana , who leers back before the two actors and the crew break into laughter . To date , the rehearsal outtakes have not been commercially released , though they are widely available on video - posting websites . " Doug Who ? " , retitled " Merry Christmas Doctor Who " , is included as a bonus feature in the expanded " Key to Time " DVD set released in 2007 in the UK and 2009 in Region 1 . DVD and video releases . DVD releases . Contents ( 2009 version ) : Disc one : Disc two : Notes :