Nardole is introduced in the 2015 Christmas special The Husbands of River Song . He was hired by Professor River Song as her companion and servant on Mendorax Dellora on Christmas Day 5343. He unwittingly brought the Doctor and River back together, mistaking the Doctor for a surgeon River had requested to extract a diamond from the head of the ruthless King Hydroflax. During the episode Nardole's head was cut off, but he managed to survive because it was uploaded to King Hydroflax's cyborg body.In the 2016 Christmas special, The Return of Doctor Mysterio , Nardole appeared as the Doctor's companion. Once his final night with River Song had concluded, the Doctor was somehow able to reattach Nardole's head to his own body. Nardole then joined the Doctor as a companion (and Co-Doctor ) so that the grieving Time Lord would have someone to stop him from going too far over, knowing that River was about to die at the Library.In 2017 series opener, The Pilot , Nardole escorts Bill Potts into the Doctor's university office, and makes a mechanical sound when raising his arm and dropping a bolt of some kind, suggesting that his body is cybernetic. Immediately in the next episode Smile , he is referred to as 'Mum' by the Doctor due to his controlling and bossy attitude towards the Doctor's 'oath' and guarding of the vault. He's pushed off to make tea but refuses to make any for Bill stating he isn't a slave to a human. But as he leaves the Doctor whisks Bill away on their first trip, he is unaware of this. When he returns to the Doctor's office in Thin Ice , he is outraged at the Doctor for his decision to leave with Bill, in violation of his oath to the vault. He seems to regularly work on the vault holding a loyalty to help, at the end of Knock Knock he is found once again working on it, the inhabitant doesn't respond or communicate with him, but comes to life when the Doctor arrives. The Doctor once again pushed him away, he refuses out of loyalty to the Doctor's oath but the Doctor doesn't show any sign of care.During Oxygen , the Doctor finds Nardole has seen past his erring. He lightens to Bill during the episode, helping her to save breaths but also help her understand the Doctor. He shows his anger towards the Doctor once they have returned, demanding to never see a recurrence of the past events.