It's the age-old conundrum: If you travel back in time, poke around, make different choices, take different paths, will the course of history be changed? And if you meddle in the wrong place at the right time (or vice versa), will you even exist when you return to the here and now?

Such are the questions posed by X-Men: Days of Future Past, the entertaining seventh installment in the Marvel Comics-based series about misfit mutant heroes and villains and their diametrically opposed mentors, Professor Charles Xavier and Magneto.

Directed by Bryan Singer, who presided over the first two pictures before ceding the series to other hands, Days of Future Past represents a geeky total immersion into X-Men mythology, beginning in a post-apocalyptic near-future ("a dark, desolate world," with Swiss cheese skyscrapers jutting over what's left of the New York skyline) and then toggling back to 1973.

And toggling back with witty, Mad Men-esque attention to period detail: When Logan, the mutant also known as Wolverine - played in both past and present iterations by the ageless Hugh Jackman - awakens in a Greenwich Village apartment, the first thing he eyeballs is a lava lamp, the first thing he feels is the slosh of the water bed he's on.

Wolverine's mission, should he choose to accept (no choice, he already has), is to take Mr. Peabody's Wayback machine - wait, no, to teleport his mind and mutton chops back 50 years, aided by Ellen Page's phasing-through-matter mutant, Kitty Pryde, and try to stop Mystique, also known as Raven, from killing a scientist, Bolivar Trask (a bearded, bespectacled Peter Dinklage). Trask has "weaponized" Mystique's DNA and her mutant powers, turning them into an army of "Sentinels" that will go to war against the mutants, pretty much exterminating them and everyone else on the planet.

Further elucidation is futile: Either you get this stuff, or you don't. But it's nice to see flashes of humor popping up, such as the glimpse of a vintage Star Trek episode on TV, with William Shatner's Captain Kirk facing his own time-travel quandaries. Suffice to say, several matters of historical significance are addressed in Singer's movie: There's a new twist for JFK assassination conspiracy buffs to consider, and President Nixon emerges in a whole other light.

Jennifer Lawrence, in case you hadn't heard, is Mystique. Painted head-to-toe in blue when she's not shape-shifting into someone else, the actress gets to show off her, um, chops, assuming various striking martial-arts stances, speaking Vietnamese, squinting her yellow cat-eyes menacingly. Another Oscar nomination? Not likely.

Lawrence's X-Men: First Class colleagues James McAvoy and Michael Fassbender are back, too. The former is the young, still-ambulatory Charles Xavier, his telepathic abilities tempered by, well, a bad temper and a boozy cynicism. The latter is Magneto, his genius nemesis, imprisoned in the bowels of the Pentagon. Later on, of course, they turn into Patrick Stewart and Ian McKellen, respectively.

Ambitious, even audacious, the movie's mix of action and for-devotees-only intrigue can overwhelm, but there are moments of sheer virtuosity, too, perhaps most notably a scene in which the supersonic Quicksilver (Evan Peters) meticulously rearranges the bullets and bodies in a slo-mo conflagration - to the tune of Jim Croce's "Time in a Bottle."

Nothing sloppy about that: Croce's hit was released in the fall of 1973, just about the same time as Nixon and the X-Men crossed paths.