Some of the details of the Terminator: Genesis plot have been revealed, sort of.  The thing about any time travel scenario is that you inevitably have to deal with multiple timelines should a singular event change the course of history.  And that appears to be exactly what Genesis is addressing.
So let’s look at what we know about the new “Terminator” movie – now officially called “Terminator Genesis” (I’ve had that confirmed, too) – which is about to get underway in Atlanta and New Orleans. : It’s cue out in 2015. It’s going out through Paramount. Alan Taylor, of “Thor : The Dark World”, is directing. Arnold Schwarzenegger is starring, reprising his ‘Terminator’ role, alongside Emilia Clarke as Sarah Connor, Jason Clarke as John Connor, and Jai Courtney as Kyle Reese. And though it’s not a remake, nor will it brush over the events of the previous movies, the film is described as a fresh ‘reboot – just like JJ Abrams’ “Star Trek” (2009) was, with Schwarzenegger’s title character the connecting tissue, in the way Leonard Nimoy’s ‘Old Spock’ reprise was to that movie.

Our source, however, says a better comparison title – than Abrams’ “Star Trek” – is “Back to the Future Part II”. By that, he/she means that while “Genesis” encompasses an original plot, it also – like “Back to the Future Part II”, which saw Marty and Doc having to revisit the events of 1955 again for the film’s last quarter – returns to and revisits some of the most memorable moments from the first two “Terminator” movies.
Source: Moviehole