‘Dark Water’ introduces the return of the Master, newly regenerated into the female form of Missy. ‘Death In Heaven’ reveals more about what she has been up to since we last saw the Master in ‘The End of Time’.
Missy is aware that the Doctor saved Gallifrey in ‘The Day Of The Doctor’ so we can assume there is a link between the timelocked realm in ‘The End of Time’ and which ever dimension the planet was sent to.
This means that Rassilon and the mysterious women who meant so much to the Doctor are also in the same dimension. This could have interesting consequences if the Doctor does ever find his world as Rassilon probably won’t be on the best of terms with him.
The Master was dying when we last saw him but we don’t know the circumstances in which he regenerated. Could it have been a punishment? If so maybe his new form was specially chosen for him. Alternatively the effort of escaping might have forced him to regenerate.
It is implied that Missy has a TARDIS so upon escaping she was free to discover the current state of the universe. Since her imprisonment it had been rebooted a few times, meaning that there would be plenty to discover. PCs might encounter her during this early scouting missions.
Her stated goal at the end of ‘Death In Heaven’ is provide the Doctor with an army so that he can wage his war for good. While carrying out this task in the most twisted of ways, slaughtering the Doctor’s beloved ‘pets’ in the process Missy does seem genuine in her goal.
‘The Sound of Drums’ indicates that the Master was traumatised by the events of the Time War. His manner indicates that he feels shame for running away. The Master felt fear and tried to hide.
Upon taking over the Earth he plans to launch missiles against other alien worlds. This indicates that the Master is lashing out, perhaps still fearful of what aliens could do to him. He probably wouldn’t feel safe until every other major power is eliminated.
In ‘End of Time’ he turns everyone in the world into a copy of himself, more through circumstance than planning. Still it shows his desire for a loyal army who can protect him. It might also speak to his loneliness.
If Missy found out about the events on Trenzalore she might be horrified to learn how the different races of the universe spent so long just to prevent Gallifrey from returning. Worse these races had time travel and had attacked the Doctor’s own time stream.
Missy would know that if her existence were discovered she’d also be a target. Beyond simply selfish reasons her feelings of friendship with the Doctor could have been rekindled both due to his efforts to save their home world but his long service on Trenzalore.
The Doctor would be the only one who Missy would trust to do the right thing, saving both herself and her people. She’d know that he’d never accept that power willingly so it would have to be forced upon him.
The Master has a history of working with other alien races only to be betrayed. The Cybermen would make an ideal partnership but she might prefer ones that were loyal t her and not their Cyber Planner.
The Cybermen appear to be the same model as the far-future versions encountered in ‘Nightmare in Silver’, with some upgrades. All those featured in ‘Dark Water/Death In Heaven’ appear to be those taken from the dead, running on a program that Missy designed. It is therefore likely that she took their technology and that the real Cybermen might disapprove of her actions.
Using a slice of the Matrix Missy uploaded the minds of the dead and then downloaded them into upgraded bodies. This process could be done remotely so we might suppose that her TARDIS was using its telepathic circuits to transfer the minds of anyone who died within its vicinity.
Did Missy have the Gallifreyan technology when she escaped or did she build it herself? If it is her own creation it suggests she is more technically skilled that previous incarnations have indicated.
At some point Missy introduced Clara to the Doctor in ‘The Bells of Saint John’. This of course occurred prior to Gallifrey being saved but Missy does say that she has been up and down his time stream.
We might suppose that she exited into a timeline in which Clara and the Doctor didn’t meet but in which the three Doctor’s still found a way to save Gallifrey in ‘The Day of The Doctor’ or that those events occurred but the 11th Doctor and Clara were from a (at that time) alternative timeline.
Did Missy realise that she was creating the Impossible Girl who would be responsible for saving the Doctor throughout his many lives? Regardless of her intention Missy can take some credit for saving the Doctor.
Her end goal of this association was so that the Doctor would investigate the afterlife, with Missy hoping that Clara would be enough of a control freak to compel him. Presumably Missy would only launch the max conversion of humanity once the Doctor actually arrived.
This could explain why she had been collecting souls in the past and the future. She couldn’t be sure exactly when the Doctor would intervene and so had to be prepared for any era.
What would Missy have done if the Doctor had accepted her gift? She seems to imply that the clouds wouldn’t kill the rest of humanity if he accepted the power. Would she have been satisfied to watch the Doctor wage his war for good?
Once defeated Missy offers to take the Doctor to Gallifrey, revealing its co-ordinates. She doesn’t get the chance and she apparently lied but what would she have gained from this? Would she have just been amused at the Doctor’s disappointment, effectively undoing all her work to win his friendship back?
Is it more likely that Missy thought she was telling the truth. She only states that those where Gallifrey’s last co-ordinates. It could be that the co-ordinates had changed since her escape. The Time Lords might also have planted false memories to prevent their location being discovered by their enemies (and who would trust the Master).
Missy is apparently killed but that has rarely meant much to her previous incarnations. PCs could encounter Missy again, coming up with a new plan. Would she have given up trying to put the Doctor in charge or would she be looking for other ways to give him the power he needs to safeguard the universe (for example reassembling the Keys To Time).
PCs will probably try to stop her but what if they agree that the Doctor should be in charge? Can they find a way to work with her and prevent unnecessary deaths?