She became the first woman to play the Master, the Doctor's greatest Time Lord enemy, in
Doctor Who (1963) or
Doctor Who (2005). It was also the first time in the series that a Time Lord had become a Time Lady. Many believed her casting as Missy was done to pave the way for the part of the Doctor to be recast as a woman for the first time. This was proved true with the casting of
Jodie Whittaker as the 13th Doctor in 2017.