Last time we left the Doctor, Martha and Jack, the recently regenerated Master had stolen the TARDIS and left them stranded in the distant future. However, fortunately for them, the Doctor is able to repair one of Jack’s gadgets-a broken “vortex manipulator” (A form of cheap time travel), and is able to transport them back to the 21st century.
immediately the trio try to figure out where the Master is, and what his plans are this time. However, he’s very much in plain sight-the Master is, in fact “Mr. Saxon”-full name Harold Saxon-and he’s just been elected Prime minister of Britain. Plus, he’s married!
As to how his arch-enemy could ascend somehow to the highest office in Britain, with support from Sharon Osbourne, among others, initially confounds the Doctor.
However, he soon figures out that the Master’s up to his old hypnotic tricks, except this time on a larger scale, using satellites broadcasting a hypnotic signal, the “Archangel” network, to basically make people vote for him. “Saxon” also reveals that he’s made contact with an alien race known as the “Toclafane” (Who the Doctor knows is a time lord boogeyman name), and will make first contact with them. During a phone call with his old enemy-in which the Master learns that Gallifrey is gone, and explains to the Doctor he became Yana to hide from the Daleks- He labels the Doctor, Jack and Martha public enemies, and kidnaps Martha’s family.
In hiding, the Doctor and co. work on a way to counter the master’s plan and reveal to the world who he really is, by creating a sort of perception filter device. The Doctor also reveals the origin of the Master.
When young, the Master was sort of given a time lord initiation right, to stare into the open time vortex-the “untempered schism”, as a sort of test. However, the ritual caused the Master to constantly hear the sound of drums in his head, driving him insane.
The Doctor and co. transport to the helicarrier The Valiant, where “Saxon” is expected to meet the Toclafane. There, the Master is reunited with the TARDIS-but it’s been modified by the Master-corrupted into a “paradox machine” for a nefarious purpose.
During the ceremony, the Master’s true plans are revealed. He exposes the Doctor, Martha, and Jack before they can get the filter on them, using his “laser screwdriver” to temporarily incapacitate Jack, and using a built in Lazarus tech ‘app’ (using the Doctor’s old hand) to age the Doctor a hundred years (and without the ability to regenerate). Martha-despite her friends and family in peril-is able to escape, but vows to come back for them.
Using the paradox machine, he unleashes the Toclafane on the world, decimating the population. Lucy-corrupted by her husband-absolutely loves the carnage.
With earth royally messed up, and him in complete control-with his enemy helpless and under his heel-it appears the Master has finally won for good this time.
Or has he? There’s still one variable out there-Martha.