Talk about it here.
A long time ago on a planet, the Doctor was given the duty of executing the Time Lady known as Missy, 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. After letting her live, the Doctor had to take care of her for 1000 years, after declaring an oath. (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 were chased out of the Doctor's study, by a puddle of sentient oil which had taken on the image of a former student, the Doctor and Nardole, with Bill in tow, checked on the vault. They only left their post once they realised that the threat was after them and showed no interest in the contents of 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, hiding his fear, asserted to Missy that he would still stand guard and would not let her escape. (TV: Thin Ice)
After saving Bill and her friends from a creaking house filled with Dryads controlled 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 started playing Für Elise on a piano, 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)
Outside the vault, while standing guard, the Doctor received an email on his sonic sunglasses, from a virtual Doctor, and asked Missy, though the door, for her help. He also admitted that he was blind. (TV: Extremis)