Thin Ice ( TV story ) Bill repeats many of the concerns that Martha Jones expressed in TV : " The Shakespeare Code " word for word , such as the fact that she has arrived in a time period when black people were still enslaved and treated poorly . Unlike Martha in that episode , Bill experiences racism from Lord Sutcliffe . Bill also initially thinks that time travel worked like the movie " The Butterfly Effect " , where she could erase herself from existence by the smallest change to history . Also a change from that story is that the companion wears clothing appropriate to the era . The Doctor did not run into his previous incarnation , who had brought their wife River Song to the last Frost Fair to hear Stevie Wonder sing for her birthday , as was revealed in " A Good Man Goes to War " . It is also the first episode since 2006 ' s " New Earth " that a character has almost said a strong swear word , only for the scene to change . The Doctor ' s morality and ability to be indifferent towards people dying is also brought up again . As the Doctor goes on to explain , he has to put his outrage aside and keep a cool head to prevent even more losses . He is also shown not to tolerate racism , as he punches out Lord Sutcliffe for insulting Bill . As was the case in " Cold Blood " and " Kill the Moon " , the Doctor shows a refusal to do anything that could drastically alter the history of Earth on his own . Instead he makes his companion make the decision . Synopsis . Trying to get back to the university before Nardole notices he broke his oath , the Doctor accidentally lands himself and Bill in the past , at the final frost fair in 1814 . He decides to enjoy the unexpected trip . However , something sinister is lurking below the frozen Thames . Plot . The Doctor and Bill are shocked to find themselves on the ice of one of the Frost Fairs . Heading back inside , the Doctor moves the TARDIS onto a bridge , just in case it falls into the ice . Bill thinks they ' ve gone to a parallel universe ; however , the Doctor tells her that they travelled in time , this is London , just in the past . The Doctor is up to going to the fair , but Bill points out that they ' re in an era where her race was enslaved . He dismisses her worries , telling her where to find the wardrobe . Bill then laughs that the TARDIS has dresses . Elsewhere , a drunken man follows the lights across the ice to an area of thin ice before being sucked under leaving only his drink bottle behind . Bill steps outside the TARDIS later , dressed in period clothing . The Doctor exits next , wearing a suit and top hat . Bill wonders if there ' s anything they need to be on the lookout for , as the Doctor shuts the door . As they leave the TARDIS , they miss an alert message that a large creature is living in the River Thames . As they walk , Bill asks about being in the past ; she wonders if the future could be changed by the slightest things she does . The Doctor jokes their friend stepped on a butterfly and ceased to exist ; now Bill can ' t even remember him . Bill notes the Doctor ' s joking ; they were they only passengers in the TARDIS . As they walk , a street urchin offers them a flyer for the Frost Fair on the Thames , which they attend . Bill notes that Regency London is a lot more racially diverse than she remembers from the movies . The Doctor remarks that " history is a whitewash " , even suggesting that Jesus Christ could have another race other than Caucasian . Whilst enjoying the festivities , Bill notices strange green lights under the ice and follows them ; the Doctor reveals he had noticed them too , but let Bill have fun while she could before they get to work . The Doctor accidentally insults a pie man , who he knows cheats at a coin toss . The Doctor and Bill are approached by a girl looking for her dog that has gone missing , the girl wanting their help . However , the Doctor deduces that she is trying to con them ( he congratulates her ) , catching a boy ' s arm as he attempts to pickpocket his sonic screwdriver from behind , only for the girl to kick his leg , enabling the pair to flee . Inside their hideout , the boy , Spider , wonders what the sonic screwdriver is . Kitty doesn ' t know , but guesses it could be worth a lot of money . They are surprised by the Doctor and Bill , who give chase . However , Spider becomes distracted by the green lights , despite Kitty yelling at him for not sticking close to her . Spider is sucked under leaving only his outstretched arm , with the sonic screwdriver , protruding through the ice . Though the Doctor tries , he cannot save him and only manages to retrieve his screwdriver . His relief at having done so and his seeming lack of care towards the boy ' s death causes Bill to run off and then confront him . She demands to know how many people he ' s seen die - he doesn ' t know - and how many people he ' s killed - he doesn ' t say . The Doctor tells her that he is 2000 years old and doesn ' t have the time for the luxury of outrage otherwise more people would die . The girl , Kitty , takes the Doctor and Bill to meet the rest of the urchin gang - including the girl who handed Bill the flyer . After gaining their trust by giving the children the food the Doctor stole and reading to them , the urchins explain that a man with a tattoo of a ship on his hand pays them to lure people onto the ice by handing out flyers . Bill and the Doctor put on dive suits and walk out onto the ice . They cannot hear each other with the helmets on and the lights begin to surround Bill . She throws her lantern at the Doctor to gain his attention before she is sucked under the ice . The Doctor jumps through before it reseals and underwater discover a giant sea creature chained at the bottom of the river . Bill sees the hat belonging to the boy who was sucked under in its mouth before they come back to the surface . Back on the surface , Bill notes that the creature sounded miserable from its imprisonment . Finding the pie man harvesting the angler fish , the Doctor yells for him to stop . They stop him , learning he fishes out the angler fish that lure in food for the monstrous fish below the ice . Bill is sickened to learn the pie she ate was made out of those fish . The Doctor asks if the man ' s seen someone with a tattoo of ship . As he can ' t get an answer , the Doctor instead asks if there ' s anything else strange . Told of dredgers getting muck out of the Thames , the Doctor and Bill goes to investigate . Bill is less than impressed at the sight of mud , wondering why there as the creature as at the other end of the Thames . The Doctor tells her that the " head " is at the end ; they ' re at the other end now . Using the psychic paper under the pretense of conducting an inspection on the orders of a Lord Sutcliffe , the Doctor gets the foreman to tell him about the mucking by complementing his intelligence . The foreman tells them that the waste of the creature is more efficient than coal and can even burn underwater making it a highly valuable resource . The Doctor realises that it could be used as space fuel . When asked if he ' ll still have to work as a foreman , the Doctor half - jokes that he can assure him that he won ' t be having that job much longer . They visit the manor home of the employer Lord Sutcliffe to determine whether he is an alien . The Doctor insists he do the talking as they must charm Sutcliffe and Bill is likely to become angered due to the creature eating the children . Under the guise of " Doctor Disco of the Fairbrook Society " , the Doctor meets Sutcliffe who instantly makes sexist , classist but mainly racist , remarks towards Bill even calling her a " creature " and demanding she show " respect in the presence of [ her ] betters " . In response , the Doctor punch es him in the face , knocking the man down , determining that Sutcliffe is very clearly human . Bill agrees that if Sutcliffe was an alien " then that was pretty convincing racism . " Bill and the Doctor are quickly captured by Sutcliffe ' s minionss with a bruised Sutcliffe revealing the creature has been a family secret for years . The Doctor deduces Sutcliffe funded the current fair , as the regular one wasn ' t able to draw in enough victims for the creature to digest . Sutcliffe plans on blowing up the ice and sinking numerous victims to feed the creature . Bill is confused , as everyone will see it ; however , Sutcliffe states the people would be too stupid to notice the creature as they will stop to mourn the ice " breaking " . He claims to be moving the world forward by using this new fuel , citing that the lives of those eaten by the creature were not important . However , the Doctor tells him real value is putting an unimportant life above his own . Sutfliffe notes that the speech would have moved anyone with the slightest bit of compassion ; he then adds with an evil smirk " not your lucky day , is it ? " The Doctor and Bill are shoved into a carriage by Sutcliffe , who plans on storing them at the fair until he can detonate the ice ; not wishing to risk the duo having warned others to stay away , he plans on detonating at noon after he brings in an elephant to draw in a crowd . Sitting in the carriage , Bill wonders if the Doctor really is 2000 years old ; he wonders why she asks . With an impressed smile , Bill asks if it takes that long to be able to make a righteous speech like the Doctor did . Once at the fair , the Doctor and Bill are tied to a pole near the explosives . Bill thinks she can save them by screaming for help ; however , the Doctor tells her not to bother . She screams anyway , being drowned out by the people ' s cheers and the performers . Once done yelling , Bill asks the Doctor what they should do . He asks her to help him get his sonic screwdriver to cut through the ropes they ' re in . This also attracts the angler fish . Their guard rushes in and the Doctor hands him the sonic causing the fish to become attracted to him and suck him through the ice . Free , the Doctor tells Bill that they have two choices : let the creature suffer for centuries , or free it . Bill points out that it could kill everyone if let go . However , the Doctor tells her that it ' s her choice ; he works for the human race , and will only do something that could drastically change the future only if he ' s ordered too . Remembering the creature ' s moans of agony , Bill tells the Doctor to free it . Knowing people will be harmed , the Doctor tells Bill to save the people on the ice which she does with the help of the urchins , managing to convince everyone the ice is melting to get them off the ice causing Sutcliffe to rush his plans and detonate immediately . However , the Doctor had used his dive suit to plant the explosives around the chains of the sea creature instead , using his sonic to reloop the wires , releasing it . The creature begins to swim away , cracking the ice above it and causing Sutcliffe to fall into the water to his death . The Doctor pulls Bill off of the ice before she falls through though they are drenched by the creature as it swims to freedom without harming anyone , pleasing Bill as it is no longer in despair . Bill wonders where it will go now ; the Doctor believes the fish will end up somewhere cold and try to avoid the human race for the rest of it ' s life . However , she then wonders if they just doomed Greenland . As this is possible , the Doctor promises to check on Greenland in it ' s future to see if the fish lives there now . Later , Bill and the Doctor invite the urchins to Sutcliffe ' s home to be fed and cared for with the Doctor amending Sutcliffe ' s Last Will and Testament so that everything passes to Perry , one of the urchins . Bill , rather annoyed , notes that the Doctor couldn ' t alter the will to have Kitty as the heir ( due to the sexism and racism of the era ) . Back in 2017 Nardole enters the Doctor ' s office at St Luke ' s University . Placing the tea on the Doctor ' s desk , he notices that the Doctor is still in period garb . However , the Doctor tries passing his outfit off as his " tea - drinking " clothes . Bill exits the TARDIS , still in her period garb , revealing that the Doctor has broken his oath . . He reminds the Doctor of his vow whilst Bill is surprised that googling the sea creature from 1814 turned up no results . The Doctor explains that humans tend to overlook the inexplicable and that the Frost Fair involved a lot of day drinking . He shows her the headlines which detail Lord Sutcliffe ' s death and , despite the contestion of inheritance , Perry was eventually deemed the legitimate heir . The Doctor and Nardole soon toss a coin to decide whether the Doctor keeps his vow and remains on Earth , or Nardole leaves him alone . Using a trick he learned from the con artist , the Doctor wins and presumably goes off in the TARDIS with Bill whilst Nardole checks the vault . As he goes to leave , knocking comes from inside , Nardole refuses to let the entity out before hurrying away looking very worried . 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 " External links . " to be added "