To save the Doctor from the Death Zone during ‘The Five Doctors’ the High Council recruit the Master. We must assume that some time passes between the Doctor’s being removed from their time zone and the Master being summoned.
Did the High Council use their psychic powers to summon him as they did the Doctor in ‘The Trial of A Time Lord’ or take remote control of his TARDIS in flight? Given that the Master has repeatedly put the universe in danger, not to mention his part in the assassination of the last President, why didn’t they do this earlier?
The Master is remarkably relaxed as he meets the High Council and they with him. We later learn that the Master has his tissue compressor with him. No one shows any fear about how the others might act.
One could argue that the Master might actually be an agent of the High Council, that he acts as a renegade so he can interfere on their behalf and do things that the Doctor wouldn’t. Their full pardon could be a way to bring him in from the cold.
Borsua later claims that he is responsible for sending the Master to the Death Zone but this early scene indicates that he was against the idea. Could it be that he knew that the Master would actually try to help the Doctor, foiling his plan, or that he’d reveal his true colours and hamper the Doctors, preventing them from clearing the way to the tower.
While they offer the Master a new cycle of regenerations he seems genuinely disturbed at an idea of a universe without the Doctor. This could be because of how intertwined his own past is to the Doctor. Rather than being worried how the universe would cope without his rival he is concerned how his own past could be affected.
Without the 3rd Doctor it is unlikely that he’d come to Earth at all. It is possible that he wouldn’t rebel at all and he would have remained on Gallifrey. Maybe he’d still have left and team up with a hostile alien race, only this time the Doctor wouldn’t be there to save him when he was inevitably double crossed.
Still, the Master seems gleeful at the prospect of saving the Doctor. Much of his motivation seems to be to impress the Doctor, to prove he is the better man. It could be that the Master saw this as a way to achieve that goal and maybe give up his life of crime.
You have to feel sorry for the Master once he enters the Death Zone. The 3rd Doctor takes his official seal and leaves him stranded, dodging lightening bolts. When he meets the 5th Doctor not only is he disbelieved again but he is knocked out. When he wakes up he finds the 5th Doctor has stolen his only means of escape from the Death Zone and left him at the mercy of the Cybermen.
After all of that he still warns the 1st Doctor about the approaching Cybermen. He could have betrayed them or guided the Cybermen across the deadly chequered floor. Not only does he trick the Cybermen by having them walk into a trap he gives the 1st Doctor the clue he needs in order to get across safely.
At this point the Master is unaware of the promise of immortality offered by Rassilon’s ring. If he did know then it is possible that he allowed the Doctors to reach the tomb in order to translate the High Gallifreyan text, since he know they were one of the few that could.
If the Master didn’t know about the ring why was he hiding, waiting for the 1st Doctor to pass? Could it be that he was affected by the fear that was influencing the others. While he may have the power of hypnotism he might have no resistance himself.
Only when he overhears the Doctor’s talking about immortality does he return to his old ways, threatening him with his tissue compressor. Given that he still didn’t have any remaining regenerations this was too great an opportunity to miss.
You can’t blame the Doctor for acting the way he did but could things have gone differently if he’d trusted the Master? There is the risk that the Master would betray him the first time they met a hostile race, like the Cybermen, to save his own skin but consider that they are on Gallifrey.
The Master would have known that the only way out was to lower the force fields to allow a rescue. No matter what race he took to the tower they would be overpowered by the Time Lords.
Without his TARDIS the only chance to escape would be to steal the Doctor’s time machine. There would only be a slim chance of this. His best bet was to help the Doctor and get the High Council to agree to give him a new set of regenerations.
If the Doctor had accepted his help and the Master had kept his side of the bargain this could have been a turning point for him. Not only would he get acceptance from one Doctor but several incarnations.
With a full pardon he would be hailed as a hero, on equal to the Doctor, if not higher. The Master would have gotten everything he ever wanted. His desire for power or control could even be satisfied by filling the sudden Presidential vacancy.
This could have been a very interesting change in the Doctor Who universe. With the Master now President he might have changed how the Time Lords interacted with the universe.
The Doctor would always be suspicious about the Master, keeping an eye on his activities. If the Master asked him to perform a mission for him, which the High Council sometimes did, would he obey? Would he fight to expose the Master, hope that he had changed for th better or flee before things went bad?
At the end of ‘The Five Doctors’ the Master arguably did carry out the mission he was assigned. Only Flavia was left alive, performing duties as acting President. Was the agreement with the Master hushed up or did the High Council indeed pardon him and give him a new set of regenerations (while not requiring him to give up his Traken body just yet).
It could be that the High Council didn’t fulfil their side of the bargain until the Time War, when they resurrected the Master. If he was given a pardon it would explain why he was given responsibility of helping fight the war.
The Master had a history of being driven by petty revenge. Since his moment of triumph was undone by the Brigadier is it possible that he sought to gain vengeance? This could lead to an adventure based around a retired Brigadier and the PCs seeking to protect him.
It would also make sense for the Master to investigate the coronet of Rassilon. It would boost his already impressive abilities of mind control. Even the Doctor would kneel before him.
Since the ring of Rassilon was returned to his body as Borsua was absorbed into the tomb it is possible that the coronet was similarly teleported away. The PCs could find themselves in a race against time to find it before the Master.