Twice Upon a Time ( TV story ) Twice Upon a Time was the 2017 " Doctor Who " Christmas special . It was the final episode under showrunner Steven Moffat ' s tenure , and to date is the last appearance of Peter Capaldi as the Twelfth Doctor . Concluding the Twelfth Doctor ' s life , as teased in " World Enough and Time " and " The Doctor Falls " , this episode displayed the Doctor ' s regeneration and introduced Jodie Whittaker as the Thirteenth Doctor , the first female incarnation of the Doctor in the series ' history . After a surprise appearance at the end of " The Doctor Falls " , the First Doctor was an integral part of this narrative . Here , he was portrayed by David Bradley , who had previously played the original actor William Hartnell in " An Adventure in Space and Time " . Though so - called " classic " Doctors had appeared on 21st century " Doctor Who " before , this episode was the first televised interaction between a Shepherd ' s Bush - and a BBC Wales - era Doctor since 2007 ' s " Time Crash " . Archival footage from " The Tenth Planet " was used to bookmark the First Doctor ' s place in this story . The special also introduced Archibald Hamish Lethbridge - Stewart , credited as " the Captain " , a new member of the Lethbridge - Stewart family . Ben Jackson and Polly Wright made a brief re - reappearance in the programme for the first time since their exit in 1967 ' s " The Faceless Ones " . Bill Potts joined the two Doctors , and Captain Lethbridge - Stewart , as a Testimony glass avatar . Past companions Nardole and Clara , made similar brief appearances . The " good Dalek " Rusty , from Capaldi ' s second episode , " Into the Dalek " , made his second appearance — in Villengard , a location mentioned only once before on television , in Steven Moffat ' s second television story , 2005 ' s " The Doctor Dances " . Synopsis . As the Doctor nears regeneration , he stumbles on his younger self , also refusing to change . It takes a captain , a glass avatar and a familiar face to convince the Doctors the universe still needs them . Plot . The First Doctor nears his death after an encounter with the Cybermen from Mondas . He is freed from his restraints by his companions Ben and Polly , now that the threat is over . However , he is weak , his body beginning to regenerate . Refusing to give in , he leaves and wanders the South Pole , mumbling to himself that he will not change . Nearing his TARDIS , he hears a voice in the distance claiming to be the Doctor . He approaches the man knelt in the snow next to the TARDIS and declares himself as the " original " . The scene transitions to the Twelfth Doctor as he gets to his feet , shocked to see his past self . The Doctor is delighted at meeting his first incarnation , but realises that he is refusing to regenerate . The First Doctor is confused and dismisses his future self as just another Time Lord , until he notices his TARDIS has changed in size . The Twelfth Doctor questions why his younger self is refusing to regenerate , fearing the consequences the decision might cause . However , the snow suddenly freezes in midair , capturing the pair ' s attention , as a man dressed in World War I attire approaches them . He seems disorientated and asks the pair if they are a doctor . The Twelfth Doctor thinks the man is making some sort of joke . On Christmas Day 1914 , the soldier , a captain in the British Expeditionary Force , lies in a shell hole on the battlefield in Ypres . His revolver is aimed at a German soldier , who also has his sidearm trained on the Captain . Both men realise they will kill each other in self - defence , their respective troops far away in their trenches , unable to assist either of them . Just as the Captain prepares to accept his fate and fire , a wave of energy flows through the field , freezing everything around him . As he exits the hole , a bright light suddenly flashes and encapsulates him . He collapses to the ground in the arctic and , hearing the two Doctors discussing the static snow , he approaches them . The scene once more shifts back to the Twelfth Doctor ' s perspective . A portal of light opens in the distance , and the two Doctors approach it . The First Doctor orders the portal to reveal itself , while the Twelfth Doctor declares that the planet is protected , to the confusion of his first incarnation . However , the light quickly vanishes , surprising the Twelfth Doctor as " that almost never works . " The First Doctor returns to the Captain , telling him to enter his TARDIS , and the Twelfth Doctor coaxes him to follow . As they enter , the First Doctor is shocked by the changes to the TARDIS interior , while the Captain is shocked by the room ' s size . The Twelfth Doctor attempts to convince his younger self they are the same person by displaying that he too is undergoing regeneration . He also points out that the First Doctor ' s TARDIS is 70 feet away , and they are inside a later version of the TARDIS , hence the differences in appearance . Regardless , the First Doctor remains sceptical . The First Doctor notices the Captain ' s unease , telling the soldier to sit down and orders the Twelfth Doctor to fetch some brandy . He does so , and the First Doctor notices that there is less in the glass than he remembered . Suddenly , the TARDIS shakes violently , prompting the Twelfth Doctor to attempt to take off , only to find that the engines are unresponsive . Looking upwards out of the TARDIS doors , he sees large metal claws pulling the TARDIS upwards . Once the TARDIS has stopped moving , the Twelfth Doctor instructs his predecessor to confront their captors . A humanoid glass figure seated at the top of a staircase speaks to the Doctor , addressing him as " The Doctor of War . " The figure requests the return of the Captain in exchange for a friend of the Doctor . Much to the confusion of the First Doctor , Bill then exits one of the corridors . The Twelfth Doctor quickly exits the TARDIS to hug her , before slowly pulling away , drawing his sonic screwdriver , and scanning Bill . To her surprise , the Twelfth Doctor demands that she proves herself to be real , as he believes Bill to have sacrified herself to defeat the Cybermen . Bill insists she is her real self , explaining how Heather had saved her . The Twelfth Doctor asks what happened after Heather saved her and where Heather is now . Bill struggles for an answer . The Twelfth Doctor decides to confront his captor directly and runs up the staircase . He is slowly followed by his younger self , who questions the need to move so fast . They find an empty chair with several panels behind it . The First Doctor takes to observing the panels , commenting on their extraterrestrial origin . He turns to his older self , his eye glass dropping from his eye as he notices the Twelfth Doctor wearing sunglasses to scan the panels . They turn to the chair , where a woman made completely of glass materialises . While the Twelfth Doctor scans the panels with his sonic screwdriver , the original Doctor looks more closely . He berates the Twelfth Doctor for not noticing the asymmetrical features of the woman ' s face , blaming his use of the sunglasses instead of his eyes . The Twelfth Doctor shyly agrees , and his younger self throws the sunglasses to the ground . The Testimony proceeds to explain its desire for the Captain back in exchange for Bill , insisting the Doctors return him to them . The Captain , having heard the conversation , exits the TARDIS . He declares they let him be taken , not wanting any harm to come to Bill because of his cowardice . However , the Twelfth Doctor disagrees . Picking up his sonic sunglasses , he races down the stairs , remarking he will stop the Testimony once he learns its true purpose . The First Doctor once more berates him for revealing his plan , both firmly declaring they are the Doctor . The Testimony , in an effort to ruffle the First Doctor , reveals the history he is destined to create as a " Doctor of War " by showing how his future incarnations will engage in various battles , which perplexes him . The Twelfth Doctor presses on with his plan to escape , using the sonic screwdriver to activate the crane which pulled the TARDIS up to take them down , the team jumping onto the chains to escape as well . The Testimony refuses to let them escape , halting the chains and pulling the TARDIS back into the ship , although the team manage to jump to the ground before being pulled back in . Bill expresses concern about what they will do without the TARDIS but the Doctors reveal they have another , younger version , at their disposal . They head to the First Doctor ' s TARDIS , swiftly entering . As she does so , Bill comments on how much smaller it is , causing the Twelfth Doctor to briefly observe the windows before insisting his younger self take off " deep space , anywhere " . While in flight , the First Doctor insists Bill clean the TARDIS only for the Twelfth Doctor to distract him by pointing out the astral map . The Twelfth Doctor uses the sonic sunglasses to access the data banks noting what his predeccessor said earlier before placing the shades on the First Doctor . Befuddlfed , the Doctor asks why he must wear them only for his successor to remark on how much he loves it before taking them back . The Twelfth Doctor quickly realises the databanks are rather scarce , given the TARDIS is still young . To compensate , he sets the to land at the centre of the universe , Villengard , impressing his younger self at how easily he did such . While searching the area , the Captain is attacked by a creature with multiple little green legs but they save him , both Doctors recognising the creature as a Kaled mutant . The Twelfth Doctor insists Bill wait in the First Doctor ' s TARDIS to look after the Captain , but Bill blasts him by calling him a " stupid bloody arse " , leading to the First Doctor to threaten her with a " jolly good smacked bottom " , much to the Twelfth Doctor ' s embarrassment . Bill tells the Twelfth Doctor to come back alive , to which he replies : " Be here when I do . " The First Doctor fetches the Captain another brandy , realising that was why his successors was missing some . He soon leaves to join his older self , while Bill looks after the Captain , but when she grabs the brandy , her hand is revealed to be glass . While outside on Villengard , the two Doctors rest , discussing their respective regenerations . The Twelfth Doctor remarks they have yet to change because they are in a " state of grace " , though he himself finds his body starting to deteriorate . He asks why his younger self halted the regenerative process , unable to remember himself . The First Doctor admits he is " afraid " about regenerating . Before the Twelfth Doctor can answer the question himself , they suddenly come under attack from a creature in a nearby tower , and only stops when the Twelfth Doctor asks the creature to scan him , announcing : " I ' m dying ! " , before going into the tower . He tells his younger self to wait outside to which he begrudgingly agrees . The First Doctor observes the area , finding the remains of a Dalek casing . He inspects it though Bill sneaks up on him and begins asking why he ran away . The Doctor insists she return to the Captain but Bill insists , asking what he was running to . Interested by the question , The First Doctor responds that he left to discover why good always defeats evil . Bill suggests it might be because of " some bloke " though the First Doctor doubts such . Bill hugs him , thanking him for his efforts . However , when she pulls away , the First Doctor realises she is a glass avatar . Alone , the Twelfth Doctor meets with the rogue Dalek Rusty , who has taken refuge from the other Daleks hunting it . He remarks on how he is a " good Dalek " though Rusty continues to fire at the Doctor . The Twelfth Doctor decides to come to a truce to which Rusty agrees by disarming his gunstick . Given access to the Dalek Hivemind , the Doctor learns that the pilot and its ship , known as " Testimony " , were created on New Earth , designed to extract people from their timelines at the moment of their death , and archive their memories into glass avatars . " Bill " is one such avatar , created from her memories . Time suddenly freezes again , the First Doctor and Bill entering the room . Although disappointed at seeing no evil to fight , the Doctors agree to return the Captain to his timeline . Upon doing so , the Captain asks the Doctors to keep an eye on his family , introducing himself as Archibald Hamish Lethbridge - Stewart , an ancestor of the Doctor ' s close friend the Brigadier . The First Doctor assures he will do so , the Twelfth Doctor confirming they did . The Testimony remarks the Captain will not remember the incident and a perception filter will ensure the Doctors are not noticed once time restarts . As time resumes , the Doctors watch as soldiers on both sides begin singing " Silent Night " in their respective languages . Soon , each side disarms their weapons and enters the battle field , leading the Captain and the German soldier to withdraw their weapons and make peace . The Twelfth Doctor explains to the First that he deliberately shifted the Captain ' s timeline to the start of the Christmas truce , to ensure his life would be spared . The First Doctor is amazed at the fact that his successor altered time to save both men , leading the Twelfth Doctor to explain how " generally the universe isn ' t a fairy tale , but that ' s where we come in " and that " it never hurts , a couple fewer dead men on a battlefield . " This causes the First Doctor to finally understand what being the " Doctor of War " really means . With the Captain saved , the two Doctors observe the truce for a time and eventually shake hands . As they do , both Doctors hands start glowing with regeneration energy . The First Doctor declares that he is finally ready , but the Twelfth Doctor states that his predecessor will have to find out what the Twelfth Doctor decides to do " the long way round . " The First Doctor returns to his TARDIS and sets the controls to return him to the South Pole in 1986 . Overcome by his regeneration , the First Doctor declares " here we go , the long way round " and collapses as his TARDIS returns to the South Pole . Shortly afterwards , the First Doctor regenerates into a much younger incarnation . As the soldiers play football , draw and spend the day together , the Twelfth Doctor watches on , saluting the Captain - who briefly bypasses the perception filter , but dismisses him and returns to his trench . Now alone with Bill ' s avatar , the Doctor adamantly contends she is not the genuine Bill , only for her avatar to explain that memories are what define a person . To help him understand , the avatar restores the Doctor ' s memories of Clara Oswald while allowing him to briefly see a vision of his former companion , bringing him joy , before they are joined by Nardole ' s avatar who joins Bill in asking the Doctor not to allow himself to die . The Doctor refuses to give the avatars testimony of his life , stating he must be left alone to face his end . They respect his wish and leave after he embraces them both . The Doctor returns to the TARDIS . He laments how the more he saves the universe , the more it seems to be in need of saving . He briefly argues with the TARDIS , conceding that " they would get it all wrong without me " as he decides to regenerate , saying " One more lifetime won ' t kill anyone . Well , except me " , but not before relaying advice to his next incarnation . He collapses as he does so but manages to return to his feet . Finally ready , the Twelfth Doctor utters his last words " Doctor , I let you go " . With that , he regenerates , the process so violent that the energy ignites the console room as his face changes . Smoke fills the console room and the Twelfth Doctor ' s ring falls from a finger it no longer fits . Once the smoke clears , the Thirteenth Doctor stands in silence for a moment , before grabbing one of the TARDIS console ' s screens . Using its reflective surface , she studies her new appearance in stunned wonderment , realising that she has now become female for the first time . Her face cracks into a wide , ecstatic grin as she as exclaims " Oh , brilliant ! " at the discovery . Excited , she presses a button on the TARDIS console . Upon doing so , the TARDIS suddenly suffers multiple failures and , hanging thousands of feet above the Earth , sharply jerks causing the doors to face the ground . The Doctor tries to hang on but the doors open threatening to suck her out . As the Doctor grabs the console , the time rotor explodes sending her falling out of the ship . As the flames completely consume the console room , the Doctor can only watch as the TARDIS dematerialises leaving her plummeting towards the Earth below . . . Story notes . Filming locations . " to be added " Home video releases . DVD releases . " to be added " Blu - ray releases . " to be added " Digital releases . " to be added "