"Extremis," the sixth episode of Doctor Who Series 10, contained a number of revelations for fans. The Doctor is, in fact, still blind, but is hiding this fact from companion Bill, creepy, zombie-like monks are running computer simulations in preparation for an invasion of Earth, and finally, many #Whovian's suspicions were confirmed when it was revealed that his longtime frenemy Missy was inside The Vault, with The Doctor sworn to guard her for 1000 years.
The episode ended with The blinded Doctor, after receiving a warning email from his virtual self, asking Missy to help him fend off the coming invasion. Will Missy, formerly The Master, actually agree to help? If she does, can The Doctor really trust her? Maybe a look back at their long history will provide us with some clues.
The boys who would eventually become The Doctor and The Master began their relationship innocently enough — as childhood friends on Gallifrey. Upon entering the Time Lord academy at age eight, the boys were sent to an initiation ceremony in which they were to gaze into the Untempered Schism, a gap in the fabric of reality through which the entire Time Vortex could be seen. This was said to either inspire the children, drive them mad, or make them run away. The Doctor ran away, while The Master went mad. Both agreed that the experience was unpleasant.
Much later, it was revealed that The Master's madness was not directly caused by gazing into the Schism, but by Rassilon reaching through it to place a constant drum beat in his head. The drumming, meant as a key to one day free the Time Lord Council from the lock placed on the Time War, slowly drove The Master mad. This makes the character a much more tragic figure than originally believed, as the madness is truly not his fault.
A Man Of Many Faces
As The Master's growing madness drove him to acts of evil, his friendship with The Doctor crumbled. They did not encounter each other again until many years later, after The Doctor had been forcibly regenerated into his third incarnation and banished to Earth. The old friends faced off many times during this period, with The Master using various disguises and aliases in mostly fruitless attempts to hide his plots from The Doctor.
It is probably important to note that, during this time, The Master's plans mostly revolved around ways to build power and influence for himself, and his goal was not so much to harm The Doctor as to best him in battles of wits. In fact, the original plan for the Third Doctor's final story was to have had The Master sacrificing himself to save The Doctor's life, though the tragic death of Roger Delgado, the actor behind the first incarnation of The Master, kept this from coming to pass. Still, this is an interesting point to consider when looking back at the character's relationship with The Doctor. If an incapacitated Doctor had come to this version of The Master for help, he may well have agreed.
Fear Of Death
When The Master resurfaced some years later, he had become a much more sinister figure. In his final incarnation, with his body degenerating rapidly, this version of The Master was willing to sacrifice both The Doctor and all of Gallifrey in order to extend his own life. When that plan failed, he set about trying to steal the bodies of others, eventually succeeding with Tremas, the father of Fourth and later Fifth Doctor's companion Nyssa. A later, final attempt to steal the newly regenerated Eighth Doctor's body caused chaos on Earth.
This is clearly one of the darkest parts of the character's history. The Master's level up in evil could be attributed to one of two things — a fear of death, or the ever-growing madness caused by the drums in his head.
Upon his first appearance in the revival series of #DoctorWho, The Master regenerated, promptly stole The Doctor's TARDIS, and engineered a plot in which he, under the alias of Harold Saxon, became Prime Minister. He was in power for a year, weakening and imprisoning The Doctor, torturing Jack Harkness, and basically enslaving the family of The Doctor's companion, Martha. However, while he is cruel to The Doctor, The Master never physically hurts him, nor does it occur to him to kill him. A bad decision on The Master's part, as The Doctor ends his reign of terror soon after regaining his strength.
Again believed dead, The Master is resurrected by his remaining followers. Upon his next encounter with The Doctor, The Master finally attempts to tell him about the drumming. Eventually, the drumming is used to release Rassilon and his council from the Time Lock. Gallifrey is intended to follow, meaning destruction for Earth. Rassilon reveals that he is the one who put the drumming inside The Master's head. The Master ultimately sacrifices himself to save The Doctor and Earth, attacking Rassilon and throwing himself into the Time Lock to close it once more.
While the evil this Master did cannot be excused, he did save The Doctor in the end. This is especially important to consider when remembering that this incarnation of The Master, played by John Simm, is set to reappear some time in Series 10.
Now we come to the current, vault-bound incarnation of The Master, Missy. While still insane, Missy is not necessarily fully evil. Missy sees herself as The Doctor's friend, and while she has been known to harm or kill those around him, has never shown any desire to hurt The Doctor himself. In fact, her first plot to raise the Earth's dead as an army of Cybermen, then gift said army to The Doctor, was a misguided attempt to get her friend back.
When she next appears in the opening episodes of Series 9 in possession of The Doctor's confession dial — a Time Lord will — she aims to locate and aid The Doctor. She even enlists his companion, Clara Oswald, for this purpose. Yes, it ends with Missy trying to trick The Doctor into killing Clara, but even this came from a place of believing that she was helping him.
The question is not so much whether Missy will agree to help The Doctor, as past evidence points to the answer most likely being yes. The problem is that Missy's insanity gives her a skewed perception of what "help" actually is. In the above example, Missy believed that killing Clara would save her friend the pain of her natural death, not at all considering that causing Clara's death would cause The Doctor greater pain.
A similar scenario could play out in coming episodes. The Doctor could release Missy from the vault on the promise of her help, but Missy would easily go on to cause chaos, all while genuinely believing that she is still providing the "help" he asked for. On the chance that Missy is actually helpful, it is unlikely that she will quietly return to The Vault afterwards, causing The Doctor more trouble anyway.
Whichever way things go, the chances of an all-out win for The Doctor are looking very slim indeed.