Describe the timeline:
In 1997, the robotic system Skynet became self-aware, deduced that humans were a threat to itself, and launched a nuclear bomb to kill half of the humans. John saves little Reese from the robot. Little Reese and others have been following John in his fight against the robots.
In 2027, John leads mankind and destroys Skynet. Skynet sent Terminator back to 1984 to kill John's mother Sarah at the last moment of destruction, preventing the birth of John, the leader of mankind. John quickly sent Reese back to protect his mother Sarah.
In 1984, Reese went back in time but was rescued by Sarah and Schwarzenegger while eliminating the Terminator together. Schwarzenegger is actually the Terminator who traveled back to 1973, and has been protecting Sarah until now.
Mother John knew that Reese was sent by her son to protect her, and she felt embarrassed that she would give birth to John with Reese. The timeline changed in Reese's memory, and the future of Skynet's awakening in 1997 did not happen, and remembered that someone said that he would destroy Skynet before Skynet completed its evolution in 2017, so he convinced Sarah to believe in himself, "Keep going in a straight line", travel to 2017 together.
In 2017, Reese and Sarah time traveled to 2017, where their son John hosted them.
Schwarzenegger also ran to meet them as agreed, and went directly to kill John.
John was originally transformed into a higher-level Terminator by Skynet using nanomolecular technology when Reese crossed in 2027, and traveled back to ensure the birth of Skynet. So caught by Schwarzenegger.
Reese and Sarah got into a fight with John. Police found Reese's parents and Reese as a child through fingerprints. Sarah comforted little Reese by saying "keep going straight", little Reese fell in love with this aunt Sarah, and remembered this sentence.
Reese, Sarah, Schwarzenegger fought against John and Skynet, and successfully destroyed Skynet.
In the end, Reese ran over and told little Reese to remember to destroy Skynet before it was born. This memory led Reese to make the decision in 1984 to travel back to 2017.
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