Sara only appears in episodes 4-12 of "The Dalek's Master Plan." She begins as enemy of the Doctor but eventually switches teams and works with him. She exits the cast when the Doctor activates a weapon called The Time Destructor that accelerates time. Sara is accidentely caught in the field of the weapon and ages rapidly down to a pile of dust.
There's a rumour going around that the BBC was trying to create an American spinoff of Dr Who where Sara would be the primary character and Dalek fighter. Supposedly a pilot episode for this venture called "The Destroyers" was written but not created. This is universally denied by the BBC, Dr Who staff, and Sara so it can't possibly be true.
Sara Kingdom is played by actress Jean Marsh, born July 1, 1934. Marsh was a substantial actress in British and American TV. She won an Emmy, had two Golden Globe awards, and appeared in a number of popular shows in the 60s and 70s. She was married to Jon Pertwee from 1955 to 1960.