Themes of bigotry, discrimination and fears against those who are different have always been at the heart of Marvel's "X-Men" comic books and the subsequent film adaptations, but cautionary sci-fi/thriller "X-Men: Days of Future Past" cranks up the grim urgency of these subjects. Linking the original trilogy—2000's "X-Men," 2003's "X2," and 2006's "X-Men: The Last Stand"—with 2011 prequel "X-Men: First Class," director Bryan Singer (2013's "Jack the Giant Slayer") and screenwriter Simon Kinberg (2012's "This Means War") crisscross between two separate periods fifty years apart as a desperate effort to save the future by altering the past gets underway. The most narratively complex of the series but also the most open to plot holes within its time-travel scenario, the film engages, then truly excels during a wonderfully staged last scene that hits an emotionally true sweet spot. The more one ponders the movie as a whole in retrospect, however, the more its missed opportunities—a poorly used villain; a handful of wasted actors; a strange framework that mostly eschews a three-act structure—reveal themselves.

2023. A war against the mutant population has brought about a global apocalypse that has destroyed most of mankind. Among the survivors, friends-turned-enemies-turned-friends Professor Charles Xavier (Patrick Stewart) and Erik/Magneto (Ian McKellen) have a plan to reverse all that has been lost by using Kitty Pryde's (Ellen Page) powers to transport the mind of immortal, virtually indestructible John Logan/Wolverine (Hugh Jackman) back in time to 1973. It is here that radical military scientist Dr. Bolivar Trask (Peter Dinklage) hopes to receive funding for a line of synthetic Sentinel robots programmed to hunt and wipe out mutants. Shapeshifter Raven/Mystique (Jennifer Lawrence) wants Trask dead, but Logan, from the future, has the foreknowledge that this fateful action will be the catalyst for the world's eventual demise. Teaming up with the younger versions of Charles (James McAvoy) and Hank McCoy/Beast (Nicholas Hoult), they set out to rescue the younger Erik (Michael Fassbender) from his Pentagon imprisonment—he has been implicated in the assassination of President John F. Kennedy—and track down Mystique before it is too late.

As much a revitalization as "X-Men: First Class" was, "X-Men: Days of Future Past" feels as if it is marking the dawn of a new era, breeding the nostalgia of the older films with the freshness of its immediate predecessor. In seguing between a stark vision of Armageddon on the verge of sounding its death rattle with the tweaked semi-historical events of the 1960s and disco-swinging '70s, Singer helps to build upon Charles, Erik and Mystique's backstories. The film also gives Hugh Jackman (2013's "Prisoners") a chance to return to the stoic heart and soul of Logan/Wolverine after sleepwalking his way through dopey material in 2013's "The Wolverine." And, if James McAvoy (2013's "Trance") and Michael Fassbender (2013's "The Counselor"), as Charles and Erik, do not exactly resemble elder counterparts Patrick Stewart (2012's "Ted") and Ian McKellen (2013'S "The Hobbit: The Desolation of Smaug"), at least their performances are compelling enough to overlook this ill-fitted casting. How Stewart's Professor X is suddenly alive in 2023 when he was seemingly killed near the end of "X-Men: The Last Stand" is more suspect; it is a reckless discrepancy that could have—and should have—been explained.

Finding parallel correlations between the events in a lava-lamped 1973 and a cataclysmic landscape a half-century later where certain doom is approaching, the film's heightened circumstances grab the viewer's attention immediately. The bleak imagery in scenes set in the isolated mountains of Mongolia as the Sentinels approach in flying cocoon-like spacecrafts is particularly striking, though these futuristic segments are also the least nourished. Reprising her role as Kitty Pryde, Ellen Page (2012's "To Rome with Love") spends the majority of the movie holding her hands up to Logan's temples, teleporting his mind back in time. Halle Berry (2013's "The Call") has one memorable moment in the third act, but has nothing else to do but stand around for her few scenes. That is still more than can be said for Anna Paquin (2011's "Scream 4"), who gets high billing in the credits for what amounts to a silent, two-second walk-on. Nonetheless, it is nice to see her Marie/Rogue again, her appearance super-brief but adding to the fitting sentimentality of the finale.

Singer's handling of action set-pieces and especially hand-to-hand combat has always been on point, smoothly shot and coherently edited without any of the shaky-cam histrionics that too many of today's filmmakers fall back on. The one exception to this latter point also turns out to be a highlight: a showdown involving Mystique and Magneto on the streets of Paris shot, Zapruder film-style, through the lens of a spectator's personal 8mm camera. Also pretty technically stunning is a slow-motion kitchen shootout involving supersonically speedy new mutant Pietro Maximoff/Quicksilver (Evan Peters), inspiringly scored to Jim Croce's "Time in a Bottle." The Washington, D.C.-set climax, taking place at a Sentinel Program unveiling on the White House lawn, engrossingly brings things full circle as Mystique's decision of whether or not to take Trask's life decides nothing less than the fate of the planet.

Were it not for a dread-inducing post-credits coda, "X-Men: Days of Future Past" would almost—almost—make for a fitting, if imperfect, conclusion to the franchise Bryan Singer began in 2000. This is a satisfying entry, to be sure, and perhaps even the most ambitious, but the premise and its hulking ensemble are underused. While the blue-skinned Jennifer Lawrence (2013's "American Hustle") is consistently riveting to watch, bringing an increased agile physicality to the role, she is given little to do beyond kick butt until the end. Her character's arc was more complicated and studied in "First Class," whereas here she is gone for long stretches and doesn't interact a great deal with the other characters. As Dr. Bolivar Trask, Peter Dinklage (2010's "Death at a Funeral") is ingeniously cast as the main human antagonist, but then fades into the background. When it comes to heavies, the script doesn't give him a fraction of the material to munch on as Kevin Bacon in the previous pic. It is easy to think of all the ways "X-Men: Days of Future Past" could have been improved—the amount of paradoxes that would be created from what occurs in the '73 scenes are staggering, yet swept under the rug—but there is plenty that Singer gets exactly right. Gradually, steadily, the film builds in provocation as well as entertainment while daring to consider a world in which the so-called X-Men are far more fallible than they may seem.