Following his heroics in saving the world from Judgment Day, John Connor has now chosen to live in secret, avoiding anything that Skynet may track him down with. That is, until he comes face to face with T-X, a robotic assassin tasked with completing the mission that T-1000 (Updated) began. Because Connor's old adversary, the Terminator, has returned to assist the now-adult Connor... just as he promised in the last episode.
One of the reasons a Terminator 3 took so long to make is because James Cameron, like it or not, is one of those Hollywood men who has a modicum of regard for the material. He thought he had nowhere to go after Terminator 2... so the studio found a pair of people who did and commissioned THEM to write the sequel! Naturally, the film continues to feature Arnold Schwarzenegger; otherwise, no one would care to see it.
Sources