John Connor and the Resistance have isolated Skynet.
Before they succeed in destroying Skynet, Mark Connor (named after Marcus Wright who saved John Connor in Terminator 4 Salvation), the son of John Connor and Kate Brewster, is killed by Cyberdyne's most successfull prototype, the TH-1.
John Connor and a Special Resistance Mission Team, SRMT, are send back from the future to the year 2014, the year that the TH-1 Humanoïd was first produced by Cyberdyne.
If they succeed in preventing this, Mark Connor will live!
Before the rest of the Resistance can shut down the few remnants of Skynet, the programm successfully travels back to 2014, accompanied by an army of High Tech Terminator Machines and the only remaining TH-1.
John and the SRMT are forced to destroy Skynet, using only weapons they can salvage in the present day.