The Constant Doctor ( short story ) The Constant Doctor was the fifteenth and final short story of " The Scientific Secrets of Doctor Who " , featuring the Fifth and Twelfth Doctors . Summary . The Doctor , Adric , Nyssa , and Tegan arrive on Lemaria to discover they have arrived during a Lemarian holiday , Freedom Day , the day when the Lemarians were freed from the tyranny of the Megrati . The group is invited to watch a reenactment of the liberation . On the way , they spot a mural that depicts the liberation . To their shock , an image of the TARDIS is on the mural , along with the image of the First Doctor . The Fifth Doctor , however , doesn ' t recall his previous visit . He isn ' t concerned , though , as he ' s saved a lot of planets , and this incident took place a long time ago for him . The four find seats for the performance . They watch as the performance plays out . When it gets to the part about the Doctor ' s arrival , the Doctor notes that the TARDIS prop was very well done , despite not being fully accurate to how the TARDIS used to look . At that moment , the actor portraying the First Doctor steps out from the TARDIS , along with the actress portraying Susan Foreman , shocking the Fifth Doctor ' s companions , as they didn ' t know the Doctor had a granddaughter . The Doctor quickly avoids the topic by going to get some food , leaving the three companions to talk to each other , realising how little they actually know about the Doctor . They then start talking about regeneration , about how each of them was affected by the Doctor ' s most recent regeneration , and wonder about how the Doctor views the process . The Doctor returns with the food , telling them that he now remembers his previous visit . As he recalls , it didn ' t take him long to liberate the planet . There was already a resistance group , when he arrived , and all he did was give them a helping hand . The four go back to watching the play , but the Doctor is curious about how good of a copy the TARDIS prop is of the real thing . They also notice that the actor playing the First Doctor has a Scottish accent . The play ends and the actors take their final bow . The stage Doctor notices the four in the audience and smiles at them . The Fifth Doctor tips his hat to the stage Doctor , having realised the truth about the actor and the TARDIS prop . At that moment , a fleet of Megrati ships arrive , and the leader teleports onto the stage , wanting to know who summoned him here . The stage Doctor steps forward , revealing that he had detected the invasion fleet on its way , and had sent the invitation so that he could have a chat with the leader . The Megrati leader tries to attack the stage Doctor , but the man disarms the leader . The stage Doctor tells the Megrati leader to take his fleet and leave , or have his ships destroyed . The Megrati leader doesn ' t believe that the stage Doctor can stop him . However , the stage Doctor pulls out a sonic screwdriver and uses it on the crystals in the Megrati command ship ' s engines , destroying the ship slowly enough to allow the crew to evacuate to other ships . The use of the screwdriver is enough for Nyssa to realise that the actor is no actor , but is an actual incarnation of the Doctor . The Twelfth Doctor once again demands the Megrati leader to leave . Seeing no other option , the leader decides to leave . Before he teleports away , the Doctor warns him to never return . If he does so , the Doctor will wipe out the entire fleet . As the crowd watches the ships start to leave , they start shouting at the Doctor to finish the job and destroy the other ships . The Doctor turns to the crowd , and gives them an angry look , asking them if that is what they really want . He even offers to let one of them take the screwdriver to deal the killing blow . No one moves or says anything , so the ships are leaving , and so is the Doctor . The Doctor walks over to his TARDIS , telling the crowd he will go make sure the Megrati leave this star system , and tells them to get back to celebrating . Nyssa is happy to see this , knowing that no matter how much the Doctor changes , he will always be the Doctor . The Twelfth Doctor takes a final bow . With one last look and smile at the Fifth Doctor ' s group , he steps into his TARDIS and departs . References . " to be added " Continuity . " to be added "