World Enough and Time ( TV story ) The episode continued the ongoing vault story arc . Notably , the Doctor takes on her first adventure , and entrusts her with his two companion s in his attempt at testing how good she has become . The episode also showed Bill Potts being converted into a Cyberman and displayed what appeared to be the beginning of the Twelfth Doctor ' s regeneration . It also notably featured the return of John Simm as , seven years after his last appearance in " The End of Time " . The episode showed John Simm ' s Master acting alongside Michelle Gomez ' s Missy , marking the first onscreen appearance of more than one incarnation of the Master . It also marked the return of the Cybermen in their original design for the first time since they were introduced in 1966 and their first formal appearance since " Death in Heaven " . The episode depicted a fourth alternative account of the genesis of the Mondasian Cybermen - different origins have been shown in the comics " The World Shapers " and " The Cybermen " , and the audio story " Spare Parts " . Synopsis . The Doctor decides to test how good Missy has become by sending her on a trial run with Bill and Nardole . However , when things go wrong , the Doctor takes over . With Bill trapped in a different time zone , can the Doctor make it to her before it is too late , and who are all those people getting cured ? Plot . The Doctor stumbles from his TARDIS onto a snow - filled landscape , repeatedly chanting no as he falls to his knees . He begins to regenerate . Sometime prior , while walking through the courtyard at St Luke ' s University , the Doctor decides to test if Missy can turn good , despite Bill ' s objections . They discuss it further in St Luke ' s kitchen , Bill claims even Nardole will resist , but the Doctor assures he won ' t as he agreed " in his head " , Nardole sheepishly agreeing . Their short dispute causes the Doctor to become emotional , due to Missy being the closest thing he has to someone else like him , something that amuses Nardole , who takes a selfie with him to chronicle the moment . Later that day , Bill and the Doctor eat together , the latter convincing her it is a good thing to let Missy try to prove herself as she was once his " man crush " . After a brief discussion about the Time Lords and gender stereotypes , Bill confesses that Missy truly scares her . The Doctor tries to promise he can ensure Bill won ' t die , but admits it can ' t be guaranteed . Regardless , he assures her that he will be there should things get out of hand . The plan set , the Doctor drops Missy , Bill and Nardole off on a colony spaceship facing a black hole in the hopes of saving it . Missy introduces herself as ' Doctor Who ' to the cameras , which she claims is the Doctor ' s real name despite the Doctor denying such . He soon becomes impatient with their progress , walking around the TARDIS eating crisps . Missy finally follows suit by deducing why a distress call was made . She also mocks Nardole and Bill ' s roles as " comic relief " and " exposition " given their genders . They are soon met by a blue skinned humanoid named Jorj who demands to know which one of them is a human . Bill confesses , the Doctor exiting the TARDIS , taking charge as Jorj prepares to shoot Bill as creatures at the bottom of the ship begin to come up in the elevators . Despite the Doctor ' s pleas , Jorj shoots Bill in the chest , mortally wounding her . Figures with bandaged faces in hospital gowns collect her body , and , using a voice synthesiser , claim that they will fix her but shall not return . The Doctor trusts them , leaving a psychic message for Bill to wait for him when she awakes . Jorj threatens the Doctor when he attempts to scan the elevators but is threatened by Missy with her umbrella , who states only she is allowed to kill the Doctor and that it will only make the Doctor cross . Once things settle down , the Doctor explains that , due to the lower part of the ship being further away from black hole , time moves faster than the one they are in ( the top floor ) noting how lifts slowed down when they neared the top due to the difference in gravitational pull , using a red pen in his sonic screwdriver to illustrate . He also states that the creatures on board the ship are the descendants of the crew who went to the lower part of the ship when it got dragged towards a black hole and needed to be reversed , as decades have passed for them even though it has only been two days for Jorj . Jorj admits his confusion ; the Doctor mocks him for his lack of knowledge despite supposedly going to " space school " , causing the man to reveal he is only a janitor . The Doctor then knocks out Jorj using Venusian aikido so that he and the others can follow Bill . Nardole , stating it has been ten minutes for them , wonders how long Bill has been living below , to which the Doctor replies he hopes they aren ' t too late . Meanwhile , at the base of the ship , Bill awakes in a hospital ward with a cybernetic heart fused in to her chest , too shocked to look at it . A feral caretaker looks at her before running off . Coming to her senses , she briefly sees a vision of the Doctor telling her to wait for him only for him to vision . She observes the room , noticing two clocks on the wall , one for floor 1056 ( which registers 365034 days , 23 minutes , and 26 seconds ) and another for floor 0000 ( which indicates 2 days , 10 hours , 45 minutes , and 17 seconds ) . Bill eventually clambers to her feet and begins to follow the noise of someone chanting " pain " , soon finding it be another patient on the ward . She goes to fiddle with its voice synthesise but has to hide as the feral man and a nurse enter the ward , the nurse mutes the synthesiser . The caretaker , revealed to be called Mr . Razor , takes a liking to Bill and brings her to his room . He explains how time passes faster for them compared to the top , using a tin can to illustrate , and offers Bill tea . He explains how Bill was hurt on the upper levels so had to have her heart replaced with a " shiny " cyber heart and that she has been at the hospital for a few months . He also shows her a live feed of the upstairs , which Bill then spends her time watching despite its extremely slow pace . Months go by , Bill witnessing how the Doctor raised his eye brow for a week and braced to explain the situation to his associates . While waiting , Bill recovers enough that she is eventually put to work cleaning the hospital by the nurse while plagued by the Doctor ' s message to keep waiting . On one particular day , she contemplates leaving the hospital only for it to alarm the other patients on the ward , unnerving her enough to stay . After being there just over a year , she eventually convinces Razor to take her out of the hospital , where he reveals to her that the patients are being converted in preparation for " Operation Exodus " as the humans ' finite lifespan cannot survive the journey back up to the top . They witness how the dying residents are guided to the hospital by the partially upgraded . After a while , Bill ' s cyber heart fails , forcing them back to the hospital . More years pass , Bill eventually convincing Razor to take her to the elevators so she can be transferred back upstairs . They sneak in , Razor revealing he has a key to the conversion theatre , wearing a mask in skit attempt to disguise himself . However , once inside , it was revealed to have been a scheme to condemn Bill to a full conversion , as ( according to the surgeon ) people usually scream if alerted to the real reason why they are taken into the theatre . The prototype patients in the room restrain her as the surgeon remarks how the headpiece he will fit on her head will inhibit emotion so she won ' t care about the pain of the process . Meanwhile , the Doctor , Missy and Nardole make it to the bottom floor , the Doctor having Missy delve into the ship ' s computer history to find out what has occurred below , annoying Nardole as that tends to be his job . He and the Doctor explore deeper into the hospital , finding a surgery room . The Doctor uses his sonic to activate the light above the bed , Nardole jumping in surprise at there being partially converted patients in the room . Elsewhere , Razor approaches Missy as she researches , the Time Lady threatening to kill him if he comes closer than three feet . The others eventually learn of " Operation Exodus " , while Missy simultaneously learns the ship came from the planet Mondas , twin planet of Earth . Razor teases her again , drawing a pistol , agitating Missy enough to confront him directly , but he discards the weapon . Razor states that he is " worried about his future " and is surprised Missy can ' t remember being on the ship beforehand . He talks to Missy about his love of disguises , wondering if she still likes them , revealing " Mr . Razor " was an invention so that Bill did not recognise him as a former prime minister of Britain . He removes his mask , revealing himself as , in his " Harold Saxon " incarnation , now sporting a beard and grey hair . He greets his future self , smiles and says " give us a kiss " . In the surgery room , the Doctor opens a door , revealing a complete Mondasian Cyberman . Backing away , he confirms he means no harm and asks for Bill ' s location . To his horror , the Cyberman asserts that it is Bill . Missy and her predecessor enter , the latter surprising the Doctor as they profess they have made " the genesis of the Cybermen " . Bill begins to cry beneath the mask , as the episode ends . Crew . " to be added " Story notes . Production errors . " to be added " Home video releases . DVD releases . " to be added " Blu - ray releases . " to be added " Digital releases . " to be added "