Have Excedrin handy: This post is about ‘Terminator’ continuity (Movie and TV commentary)

What happened to Derek Reese and Cameron? And why does Kate Connor look so young? The “Terminator” timeline(s) is (are) complex enough without these unanswered questions from “Terminator Salvation,” which opens today.

In the “Terminator” franchise — which includes four movies, one TV series, nine original novels (by four authors) and so many comic books and graphic novels that I won’t try to count them — we know “the future is not set.”

Sarah Connor always preached that mantra to her son, John, but she was talking about who wins the future war between Skynet and the Resistance. That war’s outcome varies between the timelines. (This Web site shows there are seven timelines, but I think that’s a conservative number. And here is another “Terminator” timeline site that sums things up pretty well.)

That having been said, the cement blocks of the “Terminator” foundation do not change (at least they should not change, if the writers are doing their job). This was the central theme of “Terminator 3: Rise of the Machines,” which ended with Judgment Day. In every timeline, Judgment Day occurs. The maneuvering of the Connors merely changes the date on which it occurs.

Another certainty, if we’re to have any emotional stake in these stories, is the cast of characters. Sarah Connor and Kyle Reese are the parents of John Connor, who is born in 1985 after the events of “Terminator 1.” Derek Reese, introduced in “Terminator: The Sarah Connor Chronicles,” is the older brother of Kyle; they grew up together and saw the Judgment Day bombs launching while they were playing ball in the backyard. Kate — also a key Resistance fighter — is the wife of John. They have a daughter named Kyla (Kate is pregnant at the time of “Salvation”).

In “Terminator 1,” John sends his young charge Kyle back in time to protect Sarah (this is another of those foundational blocks that is true in every timeline, except of course in the very first one where John must have a different dad — but we won’t worry about that one). And in “The Sarah Connor Chronicles,” he sends Derek back to protect his teenage self. He also sends back Cameron, a machine who looks like Allison, a human girl caught up in the future war.

In “Terminator Salvation: From the Ashes,” a prequel novel to the film, author Timothy Zahn has a teenage Kyle acting as a protector to fellow street urchin Star, a young girl. There is no mention of Derek (Kyle’s “protector”), which is odd, because we know the brothers grow up together. If Derek split off into a different group of people, fine, but Zahn should mention this.

Adding further unnecessary complications to the story, John Connor will be played by the fourth different actor in “Terminator Salvation.” Christian Bale follows Edward Furlong (the child John), Thomas Dekker (the teenage John) and Nick Stahl (the young adult John). Ideally, Stahl, not Bale, should be playing the adult John. “Salvation” is set 15 years after “T3” (2018, compared to 2003) and Stahl has aged six years since that movie was released, but I say that’s close enough, especially considering my next point.

In “Salvation,” Kate Connor is played by Bryce Dallas Howard, who is 28 years old. Claire Danes, who played Kate in “T3,” is 30 years old. “Salvation” takes place 15 years after “T3,” so if we assume that Kate was 24 in “T3” (Danes’ age at the time), then Howard is playing a 39-year-old in “Salvation.” I’d buy Danes in the role alongside Stahl, but if casting an older John was important, why wouldn’t you cast an older Kate, too?

At some point in the “Salvation” trilogy (two more post-Judgment Day films are planned), we’ll arrive at another one of those core plot points: John sending Kyle through the time bubble. I assume this will happen at the end of “Terminator 6,” since it would be a way to tie everything together in a (sort of) neat bow.

I’m sad to see that Fox canceled “The Sarah Connor Chronicles” (odd timing, too, with the movie coming out), but if it had continued, it would’ve muddied the continuity even further. The series ended with teenage John jumping to the future (again), where he meets Derek and Allison (for the first time, from their perspective, because they had been waiting for someone named John Connor to emerge as their savior).

Such a scenario doesn’t fit with what we already know — Future John should be at least a decade older than both Derek and Kyle. As much as I enjoyed “Chronicles,” the show’s introduction of forward time-travel really made the timeline confusing.

I wrote a column last year attempting to explain the “Terminator” timeline. Basically, every time someone from the future pops up, a new timeline begins. The new “Star Trek” movie illustrates one instance of timeline creation. Everything from the previous TV shows and movies happened, then Spock traveled through time, and now everything is happening again, in a new way.

However, the core elements (Kirk being a womanizer, Spock being logical) are still in place. It’s one thing to talk about the future not being set, but it’s another to say Kyle Reese has a brother in one timeline and not another.

If timelines are that incredibly fluid, then it raises one final question: Why are we following the “Terminator” franchise if everything we see or read can be erased by the next thing we see or read?