In the story, it’s been ten years since a man-shaped killing machine (helpfully described as a “cybernetic organism: living tissue over a metal endoskeleton”), played by Arnold Swarzenegger, was sent back through time to “terminate” Sarah Connor, played by Linda Hamilton. Sarah’s offense? Being the future mother of John Connor, who’s going to lead the rebellion in a future war against the machines, after a mysterious neural net-based artificial intelligence group called Skynet accidentally paves a way for its own creation to annihilate humankind.
As Sarah herself explains in the voiceover to Part Two,
Three billion human lives ended on August 29, 1997. The survivors of the nuclear fire called the war Judgment Day. They lived only to face a new nightmare: the war against the machines.”