I’m not sure there is a stranger big-screen superhero franchise than the “X-Men” movies.

I’m not counting the new series of “Spider-Man” movies, which Sony is apparently trying to expand into an entire universe by basing movies on villains and second-string characters. You think the general public hadn’t heard of Iron Man before 2008? Try basing an entire movie on Black Cat or Venom.

And I’m also not talking about the “Fantastic Four” reboot, which seems alarmingly intent in removing everything “fantastic” from the story, characters and situations of Marvel’s First Family.

Heck, I’m not even talking about “Ghost Rider,” which is inherently weird.

It’s just that, since 2000, the “X-Men” movies have followed an oddball path. Director Bryan Singer made two good movies – I’d even say that the first sequel, “X-Men United,” was a great superhero movie – then left the series for the unfortunate “Superman Returns.” Some “Wolverine” spin-offs ensued which gave us charismatic Hugh Jackman and little more. Matthew Vaughn’s “X-Men: First Class” was a terrific return to form, showing the origins of Professor X and Magneto and Mystique and featuring a charismatic cast in a tale of the swinging 60s and the sudden appearance of mutants in the world.

So I wasn’t quite sure what to expect with Singer’s return in “X-Men: Days of Future Past.” He brings back nearly every character and actor from the earlier films, either through starring roles, cameos or flashbacks. He introduces others, most notably Peter Maximoff, the mutant known as Quicksilver. (Quicksilver is the only character so far appearing in both the mainstream Marvel Cinematic Universe and these offshoots, making appearances here and in the tag at the end of “Captain America: The Winter Soldier” and in next year’s “Avengers: Age of Ultron.” He’s by a different actor in the MCU films.)

The story, loosely based on a classic tale from the comics, opens in a future time when a war between mutants and Sentinels – mutant-hunting robots created by Bolivar Trask (Peter Dinklage) – has ravaged much of the globe. A few X-Men, including Professor X (Patrick Stewart), Magneto (Ian McKellan) and Wolverine (Jackman) come together with a plan to change history: If they can stop young Mystique (Jennifer Lawrence) from killing Trask back in 1973 – thus speeding up the Sentinels program – they can stop the war.

To stop Mystique, they task Kitty Pryde (Ellen Page) with sending Wolverine’s mind back into his 1973-era body. There, he must find young Charles Xavier (James McAvoy) and young Eric Lehnsherr (Michael Fassbender) at a time they are decidedly not filling their fated roles as Professor X and Magneto and persuade them to help.

The plot is reminiscent of how the older Spock sent Kirk on a mission to win over young Spock and become a team and at least offers a better explanation for why a simple sit-down between key players couldn’t have resolved matters much more easily.

The movie does a pretty good job in immersing its story and characters in 1973, with period-appropriate clothing and jokes about “all three” TV channels – and PBS – and events and figures from the day, including a healthy supporting role for Richard Nixon.

The movie is relatively light on the “future” sequences, where Storm (Halle Berry) joins some younger mutants in fighting off the Sentinels until Wolverine’s consciousness can complete its mission.

The light-hearted tone of “First Class” is mostly gone. There are moments of humor – most of them involving Wolverine – but the tone is one of urgency and distance, with Charles, Erik and Raven all blaming each other for the estranged relationships among them.

The movie’s got big battles and impressive special effects, but what stuck with me after seeing the movie is the ending, that’s full of warmth and hope and could certainly lead to more stories featuring the “classic X-Men” cast. But I have the feeling the future of the franchise rests with the younger actors.
On that count, we’re left with a much murkier picture of the future.

Random observations:

I was somewhat surprised by how much Wolverine seems like a spectator in this movie. The storyline really focuses on young Charles, Erik and Raven.

One thing the movie does not have: Any sense of an explanation as to how the storyline follows the tag at the end of “The Wolverine,” which had Xavier and Magneto meeting Logan at an airport with an urgent mission … that they apparently wait a few decades to dispatch him on.

The movie has some fun cameos from familiar faces and a post-credits scene that is mystifying but apparently points toward a sequel Singer has announced based on the “Apocalypse” storyline from the comics. Who will fill out the ranks of the mutants remains to be seen.