Excerpt from Wikipedia article
http://en.wikipedia.org/wiki/Terminator_SalvationOn May 23, 2008, Warner Bros. released the following plot summary for the film. The synopsis was modified on June 11, 2008:
Set in post-apocalyptic 2018, John Connor (Christian Bale), the man fated to be the leader of the human resistance against Skynet and its army of Terminators, and the future he was raised to believe in is altered in part by the appearance of Marcus Wright (Sam Worthington), a stranger whose last memory is of being on death row. Connor must decide whether Marcus has been sent from the future or rescued from the past. As Skynet prepares its final onslaught, Connor and Marcus both embark on an odyssey that takes them into the heart of Skynet's operations, where they find out a terrible secret that may lead to the possible annihilation of mankind.[2]At the beginning, Connor does not start off as leader of the human resistance, but he will work his way up through the ranks in the film. He is also mistrusted by other soldiers due to his extensive knowledge of Skynet.[3]
McG said that it will be about the development of the Model 101 Terminator as well:[4] scenes involve humans being captured and studied by Skynet in order to perfect their cybernetic organisms and Connor explaining "If we let these things go online, the war is over.[5]" He explained the film will "begin again very much in the spirit of what [Christopher] Nolan did with Batman," in that it will not require audiences to have seen the previous films, but it also acts as "a bit of a sequel and a prequel because it tells the story of how we got there".[6]