The Plot
what we didn't know in t1-2 is that skynet had sent another t-800 back in time, as a backup/sleeper. so after the events in t2, when sarah and john thought skynet was prevented and they were save, the sleeper t-800 becomes active and catches john off-guard. john gets terminated, but sarah manages to survive and can even catch and re-program the t-800.
sarah knows that as john doesn't exist anymore, someone else will have to become the human resistance leader in the future and she regards it as her mission to train that person, like she had been training her son.
in mexico, two characters from the future arrive: a t-800/t-1000 mixture which mission it is to hunt and terminate a young woman called dani ramos. the other character is a cyborg called grace, sent to protect dani. sarah takes notice of this and sets out to protect and train dani (the supposed new future resistance leader), accompanied by her helper, the re-programmed t-800, as well as grace. after a big hunt throughout mexico involving helicopters, the showdown will take place at a dam, involving some spectacular underwater actions scenes.