The fun thing about fantasy films with time-travel plot-lines is that you can undo the mistakes of your predecessors. The first batch of X-Men movies came to a premature end with the third movie, which managed to kill off all the characters you could possibly feel any interest in except for Wolverine, who was then left to carry the franchise all on his ownsome.

X-Men: First Class was an excellent prequel - fresh, funny, brilliantly weaving itself into actual historical events, with immensely charismatic turns from outstanding actors such as Michael Fassbender, James McAvoy, and Jennifer Lawrence. This second outing for the 'youth' version of the cast is perhaps a bit overburdened with crash-bang FX in overlong fight scenes, but really there is little to complain of and I can't see why the papers were so sniffy about it. It's a very long film, but it's dramatic, suspenseful, funny, and contains well-spaced gems to delight Marvel aficionados. And because of the time-travel plot, it enables us to simultaneously enjoy the older and younger versions of the main characters. Ian McKellen AND Michael Fassbender - what's not to like? The only character to play his older and younger self is the allegedly self-rejuvenating Wolverine, but here I do have a quibble - in the fourteen years since the first movie it's clear that poor Hugh Jackman has been subjected to a regime almost as tortuous as having his skeleton replaced by adamantine, with the result that he is now so 'ripped' that he looks as if he's about to explode and, I dunno, that amount of bulging, veiny, hairy flesh is just not cool or sexy. But it's well worth a visit to renew your acquaintance with the rest of them.

The X-Men mutate into The Matrix and The Terminator in Days of Future Past. Bryan Singer's back at the helm in this second back-to-the-beginning adventure which brings Dr Xavier face-to-face with his younger self.

The X-Men are minutes from extinction in a future where seek-and-destroy robots have been designed to hunt them down. Developed as an uber-weapon back in the 1970s, by the power hungry Dr Bolivar Trask (Peter Dinklage, Game of Thrones), the bots threaten the world itself. Besieged, it's time for another last stand. Dr Xavier and Magneto persuade Kitty Pride (Ellen Paige) to send Wolverine's consciousness back in time to his 70s body. Mission? Find and persuade their younger selves to put aside their enmities and eliminate Bolivar Trask before the bots are born.

Opening with a rip-roaring, time-ripping fight, X-Men: Days of Future Past plunges you straight in. Iceman, Bishop and Storm – and all the other X-Men die-hards – battle their robot nemeses in a rear-guard action to the death. It's an eye-popping, inventive spectacle, one of several popcorn-forgetting sequences that make Days of Future Past well worth a watch. If you've seen the others, you'll relish the references. But no worries: the franchise folk have done their job of opening it up to newbies.

Better than just bang-crash spectacle, the action's tied-in to the characters; you feel there's something at stake like you never do with Spidey or Superman. Not all will live to tell the tale. A standout scene has Dr Xavier and Wolverine trying to break the younger Magneto/Erik out of a bunker: cornered, it's time for the cocky, leather-jacketed Quicksilver to whizz round redirecting the bullets and punches, in a bravura slo-mo ballet to the strains of Jim Croce's 'Time in a Bottle': the denouement is funny and thrilling.

Raven/Mystique (Jennifer Lawrence) is also a strong point: whether ass-kicking a barrack-room of blokes or axe-kicking in a board-room, she carries the action with aplomb. Hard to do in a tight-fitting blue body-stocking. Harder still to express the hunted-look when a friend becomes a foe or to convey the crisis of pursuing her own vengeance against Trask. But Lawrence ably provides the film's dramatic edge.

Xavier's nose-to-nose with his future self, catnip to afficiandos of the series, is a touch underwhelming; though James McAvoy acquits himself well opposite renowned thesp and Star Trek captain Patrick Stewart. It was McAvoy's first day of filming and Stewart's last. It functions but lacks frisson.

Yet for all its inner-geekness, there's the feeling it's been done, and done better, before. The Matrix introduced the slo-mo bullet time concept – though admittedly not with a Jim Croce song. It introduced sentinels – mechanised creatures homing-in on the trapped heroes. It also gave us the down-the-line despatch of consciousness into another state of being. X-Men: DFP owes The Matrix big-time.

So too The Terminator-esque send-back in time to avert an invention. Even Twelve Monkeys had a similar end in view. Yet it's the ending that will have fans talking. If you play with time, anything can happen. But does it add to - or undermine - the series so far? Big questions. And where does the sequel go from here? Catch the end-of-credits teaser and you may have a clue. Or not.