The Master escaped the Time Lock by falling through the Schism with the assistance of the Doctor’s Mother who came to take her turn in the Master’s eternal punishment for failing to free Gallifrey from the Time Lock. He fell through the Rift in 21st century Wales and was picked up by Torchwood, where he was rehabilitated to keep himself busy by working through the Torchwood Archives. He now travels with his Doctor.
While in the Time Lock, he was renamed the Prisoner of Rassilon, and it was Rassilon himself that patched his my being and regeneration cycle, trapping him within the same body for the rest of his life. This allowed him to always carry the memory of his ‘betrayal.’
Some time after travelling with the Doctor, he was stalked and kidnapped by the Harrier. This led to a little over six years of more imprisonment where he was used against his will and often neglected.
The Master likes things to be clean, does not like to be touched, and still isn’t all that fond of what he considers to be lower races. The effects of his more recent past have taken a toll on him and he often questions why he longed for immortality. He has given himself about 500 years of life, at which point he feels that he will pass on for the final time. Which is why he feels that he has to make the most of the time that he’s given himself.
He has taken over Saturn and turned it into a work colony, where workers live on the planet’s moons and are transported to and from work on the planet itself. They work in warehouses for very long hours, but due to the Doctor’s prodding, they at least have the basic necessities.
This was only allowed by the Master because he and the Doctor that he lives with were married. They had a bonding ceremony, and the Master thought that it was more of a way of stating a commitment. He thought it good since he has no intention to vacate their TARDIS any time soon.
So what’s next?
Only more plans being put into action and travelling are for certain.