Season eight of Doctor Who opens with a few changes. Yes, the Doctor is still trapped on Earth but he's got a new enemy to fight. A Timelord who calls himself the Master arrives on Earth. His Tardis is fully operations, complete with a chameleon circuit, allowing his Tardis to blend in with the circus he has landed in. Next to the Master is his Tardis in the shape of a horse-box.
While marooned, the producers wanted to give the Doctor a common enemy to fight. The Master became a sort of Moriarty to the Doctors Sherlock Holmes. But the Timelords have not completely left the Doctor in the lurch, one Timelord in particular arrives briefly to warn the Doctor of the Masters arrival. The Doctor isn't too worried, but he's warned the Master has learned a lot since the Doctor last met him. It's implied the Doctor knows who the Master is, but this is the first on-screen meeting of the two.
Along with a new enemy to fight, the Doctor gets a new assistant; Jo Grant. The Brigadier has hired her to replace Liz who has gone back to Cambridge, but the Doctor argues he needs a competent scientist, and Jo is not. The Brigadier reminds the Doctor how Liz always told him he only needed someone to 'pass him his test tubes and tell him how brilliant he is'. Jo indeed fits that role nicely, when she isn't being hypnotized by the Master.
Fortunately, the Doctor is able to snap Jo out of the Masters influence and they learn what his plan is. He's sided with the Autons in hopes of dominating the Earth. He takes over a plastics factory and begins to produce dolls, chairs, flowers and even telephone cords, all made of plastic and very deadly. Dolls and chairs come to life, flowers spray chemicals to asphyxiate people and telephone cords wrap themselves around people and strangle them to death.
Among his talents, the renegade Timelord is also a Master of disguise. He disguises himself to sneak into UNIT to replace telephone cords and when he's ultimately defeated he disguises someone else to look exactly like him. But the Doctor isn't fooled by the 'Old Man Peabody' routine. The Autons are defeated but the Master gets away. Only he can't use his Tardis as the Doctor has stolen his dematerialization circuit. The Master is now as trapped on Earth as the Doctor is.
WHO: Jo Grant
PLANET OF ORIGIN: Earth, circa 1970
REASON FOR JOINING: The Brigadier hires her as a replacement assistant for the Doctor.
WHO: The Master
PLANET OF ORIGIN: Gallifrey, circa unknown
REASON FOR JOINING: Despite being from Gallifrey, the Master is a renegade like the Doctor, but wishes to 'Master' the Universe and destroy all who stand in his way, namely, the Doctor.
NEXT: Cursing the Ice