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Some sequels have such a complicated back story they should come with printed footnotes. The latest sequel in the “X-Men” series requires an encyclopedia that can be read in a fun-house mirror.

Even if you’ve seen the other six “X-Men” movies, you might need a refresher course before you see “Days of Future Past.” And the course should include 20th century American history.

Two summers ago, “X-Men: First Class” flashed back to the early ’60s, when the super-powered mutants who live in human society divided into two factions. The separatists aligned with Eric Lensherr, aka Magneto (Michael Fassbender). The peacemakers aligned with Charles Xavier, aka Professor X (James McAvoy). The battle for the future of their race was waged against the backdrop of the Cuban missile crisis.

In the spectacular new movie, the aging Professor X and Magneto (Patrick Stewart and Ian McKellen) are allies in a fight against robots that have decimated both the mutant and human races. The only way to defeat them is to send razor-clawed curmudgeon Wolverine (Hugh Jackman) back to 1973, to stop government scientist Dr. Bolivar Trask (Peter Dinklage) from synthesizing the robots with the DNA of angry mutant Mystique (Jennifer Lawrence). The old and young versions of Professor X and Magneto will have to work together, on opposite sides of the looking glass.

If you didn’t follow that synopsis, you’ll get another chance when history repeats itself. Or you could just sit back and enjoy one of the loopier superhero flicks of recent memory. How could you not marvel at a movie that includes a revisionist explanation of the JFK assassination, a football stadium floating over the White House and the sight of Richard Nixon firing a .45 at a villain in a Christ-figure pose?

And your hormonal nephew knows that Lawrence spends most of the movie doing kung-fu in a form-fitting blue body suit.

Shape-shifting Mystique gets to strut her stuff from Saigon to Paris, where she’s fated to whack Dr. Trask as an enemy of the mutants. But in the rebooted version of history, her former friend Charles sends psychic signals saying “Stop, in the name of love!”

Which raises the question: If a mutant butterfly flaps its wings at the Watergate hearings, will the future see another “X-Men” movie? The Magic 8-Ball says: We hope so.

What “X-Men: Days of Future Past” • Three stars out of four • Rating PG-13 • Run time 1:31 • Content Sequences of intense sci-fi violence and action, some suggestive material, nudity and language

Joe Williams is the film critic for the St. Louis Post-Dispatch.