The Vault (The Pilot)

A secret vault, containing the Master, was hidden beneath the campus of St Luke's University in Bristol which the Twelfth Doctor made a promise to protect.

By the 2010s, the Doctor and Nardole had spent almost seventy years guarding the vault, while also working at the university.

The Doctor and Bill Potts were chased to the location of the vault by a puddle of sentient oil which had taken on the image of a former student. At this time, both the Doctor and Nardole only left their posts when 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 Vault before being interrupted as the Master started heavily knocking the door. Though acknowledging that the Doctor had been "distracted" by Bill, Nardole, hiding his fear, asserted to the Master that he would still stand guard and would not let it escape. (TV: Thin Ice)

After saving Bill and her friends from a creaking house filled with Dryads controlled by a Landlord, the Doctor returned to the vault with food to take his shift, finding Nardole fiddling with the vault doors. The Master started playing the piano. Not long after Nardole left, the Doctor told the Masterabout his day and entered the vault. (TV: Knock Knock)