In “The Mark of the Rani”, his and the Rani’s priorities are clear. While the Master spends the whole time obsessing over the most fitting way to kill the Doctor, the Rani rolls her eyes and gets on with the actual work of trying to conquer the universe.
When she runs into the Doctor again in “Time and the Rani”, she quickly captures and takes advantage of a discombobulated post-regeneration Doctor so she can use his genius to finish her plan. But while the Master would get an absolute kick out of having the Doctor as his puppet, the Rani spends her time irritated and impatient with the whole thing.
So, if she’s been following the Doctor around time and space for a while, we can trust that this is about more than vengeance. The Master might go through Time and Space, building a virtual afterlife to create a Cyberman army to give the Doctor as a present while screaming “Notice me!”, but that is not the Rani’s style. With the titles of the final two-parter, “Wish World” and “The Reality War”, we can safely guess that the destruction of planet Earth is merely a byproduct on the way to conquering reality itself.
But there is an added twist in the tale…
A Rani and The Rani
Our first real clue to the identity of the Rani is when she starts regenerating. But this is not just any regeneration. Whatever happened to make the Fourteenth and Fifteenth Doctor’s “bi-regenerate” in “The Giggle”, it seems to have had wider repercussions, because as the Rani regenerates she splits into two.
Now what’s interesting here is that normally in any multi-incarnation Time Lord encounter, there is an established dynamic. Namely, bickering. Put Missy and the Master, or any two given Doctors in a room and they will be immediately start sniping at each other in an attempt to assert dominance. That and maybe flirting a bit.