Vault ( The Pilot ) The Vault , hidden beneath the campus of St Luke ' s University in Bristol , was a quantum fold chamber which contained . The Twelfth Doctor made a oath to protect the chamber for a thousand years . Like some other examples of Time Lord technology , the chamber was bigger on the inside . The interior resembled a non - descript room with some tables and chairs and , even though it was a sealed unit located underground , windows that allowed light to enter the chamber . A centrepiece of the chamber was a piano with which Missy amused herself . The chamber had Gallifreyan markings all around it and on both sides of the doors . History . A long time ago on a planet , the Doctor was given the duty of executing the Time Lady known as , by use of a technology able to kill a Time Lord off completely . However , the Doctor fiddled with the machine and it didn ' t kill her . The Doctor had sworn to watch over her body for 1000 years , but he did not say that she would be dead . ( TV : " Extremis " ) By the 2010s , the Doctor and Nardole had spent at least seventy years guarding a vault on Earth ( TV : " The Pilot " ) that became the prison for Missy , while also working at St Luke ' s University , the premises beneath which the vault was located . ( TV : " The Pilot " , " Extremis " ) When the Doctor and Bill Potts encountered the puddle of sentient oil in the Doctor ' s study , they and Nardole escaped in the TARDIS and landed under the university to check on the vault . The team fled the campus in the TARDIS after they learned that the oil was not interested in the vault . ( TV : " The Pilot " ) After being told by the Doctor to " leave [ him ] alone " , a frustrated Nardole voiced his frustrations to himself while standing guard at the chamber before being interrupted as Missy started heavily knocking the door . Though acknowledging that the Doctor had been " distracted " by Bill , Nardole asserted to Missy that he would still stand guard and not let her escape . ( TV : " Thin Ice " ) After saving Bill and her friends from a creaking house filled with Dryads owned by a mysterious landlord , the Doctor returned to the chamber with food to take his shift . He found Nardole checking the security of the chamber doors . Missy began to play " Für Elise " on a piano which was gifted by the Doctor , against the wishes of Nardole . Nardole was dismissed by the Doctor , who went on to tell Missy about his day . Hearing about people being eaten , the tune changed to " Pop Goes The Weasel " and the Doctor entered the chamber . ( TV : " Knock Knock " ) Despite the chamber being effectively a prison , the Doctor was able to enter the chamber to visit Missy at will . ( TV : " Knock Knock " , " The Lie of the Land " ) While outside the vault , the Doctor received an email on his sonic sunglasses from a virtual Doctor and , in a one - sided conversation , asked Missy for her help . He also admitted that he was blind . ( TV : " Extremis " ) After the Monks succeeded in invading Earth , the Doctor and Bill consulted Missy inside the vault about how to defeat them where she was playing " Gnossienne No . 1 " on the piano . At first she played a game of Hot or Cold when he attempted to find the answer himself . Then she played " The Entertainer " on the piano and then told him she had dealings with them before and that he would have to kill the lynchpin which was Bill Potts . She and the Doctor later sat in the vault and conversed , during which she was thoughtful about her past killings and murders , which she realised she actually knew the names of , the Doctor realises she might be turning good . ( TV : " The Lie of the Land " ) When Nardole was sent back to the TARDIS during a trip to Mars to fetch something for the Doctor , she took off by herself , with Nardole onboard , because she sensed danger and refused to return , Nardole had no choice but to consult Missy and bring her out of the vault to steer the TARDIS . When the Doctor found out , he told her he would have to put her back in the vault again . ( TV : " Empress of Mars " )