It picks up long after the events of Terminator 3: Rise of the Machines where we see John Connor (Christian Bale) as a soldier in the human resistance against the Skynet.
Before the first battle scene, we're privy to Sam Worthington's character, Marcus - in prison, offer his body for a "good cause".
Marcus wakes up amid the battles and finds a young Kyle Reese (Anton Yelchin). Together, they make their way towards Skynet and bump into Connor. Reese is captured by the terminators. Marcus infiltrates the machine compound and Connor invades to destroy them once and for all.
Terminator Salvation is a nice addition to the franchise. It gives you a look at some of the things hinted at in the earlier films. It also touts some of the fiercest explosions I have seen in movies.
However it lacks humanity. Which is pretty much the basis of the film. You don't connect with any of the characters and you really don't care who lives and who dies. Obviously you know Connor and Reese will survive this first film (of which the director would like to make 3). But you don't mind watching the other survivors bite it because you really don't care about them.
The movie is rated PG-13 for intense sequences of sci-fi violence and action, and language.