Terminator: Dark Fate occurs in a timeline three years after the events of Terminator 2: Judgment Day and ignores Rise of the Machines, Salvation, The Sarah Connor Chronicles, and Genisys though I am not well versed in the franchise and didn't watch the last two titles listed. The film opens 1998, three years after Judgment Day. Sarah Connor and John are in Guatemala when a T-800 appears and kills John before he can lead the human resistance against the machines in the future. This is a surprise as Skynet was supposed to have been stopped.
Turns out a different AI, Legion, was created and takes over just as Skynet did but in attempting to defeat it, the humans are the ones who will cause the nuclear holocaust. Naturally, other humans will stand up against Legion so they send back other Terminators. Sarah (Linda Hamilton) has been destroying them over the years, thanks to receiving information from an unknown sender in messages that close with "For John." In Mexico City 2020, the latest Terminator model, Rev-9 (Gabriel Luna), is sent back to kill Dani Ramos (Natalia Reyes). The Rev-9 has the ability to shed its liquid skin off its skeleton and have the two work independently, combining traits of the T-800 and T-1000.
The resistance sends back Grace (Mackenzie Davis), an technically augmented human solider, to protect Dani, so basically the story is a repeat of the first two films. With the help of Sarah, they barely stay one step ahead, leaving lot destruction and dead bodies in their wake. Thinking they have an ally in whoever is sending Sarah the messages, they go to the person and discover its the T-800 (Arnold Schwarzenegger) that killed John. After having taken on a family, marrying a woman and being a father figure to her son, and the name Carl, it learned the importance of having a purpose, so it passed on the Terminator information. Realizing running away isn't enough, they decide that fighting the Rev-9 is the only way to get things to end.
Dark Fate has great action sequences, such as when Rev-9 crashes a refueling aircraft into C-5 transport while in the air and the climatic battle at a hydro-power plant, which has action underwater and on the land so kudos to the teams involved with stunt and special effects.
Unfortunately, the story is all too familiar as it basically copies the template of the first film, and like the second film, the characters from the future are powered up to a stronger level like a video game. One wonders why the AI isn't smart enough to send more than one Terminator at a time into the past and why they only pick one person to go after. Surely, there will always be a human to step forward and lead a resistance. If time travel exists, why not go back and keep a war going or stop some key invention to the extension of human life to be invented?
I'd recommend Dark Fate for the action, and the fun of seeing Hamilton, who imbues Sarah with compelling grit, and Schwarzenegger interact, even though they have become supporting characters. However, it's too bad the screenwriters didn't create a better story for those elements to appear in.