The film focuses on the war between Skynet's machine network and humanity, as the remnants of the world's military have united to form the Resistance to fight against Skynet's killing machines. Bale portrays John Connor, a Resistance fighter and central character, while Worthington portrays cyborg Marcus Wright. Yelchin plays a young Kyle Reese, a character first introduced in The Terminator, and the film depicts the origins of the T-800 (Model 101) Terminator.

References:
https://en.wikipedia.org/wiki/Terminator_Salvation