The current domination of superhero franchises in our multiplexes isn't easy to pull off.

Every new release carries evidence of the effort it takes to eke more life out of the same characters, again and again.

Spider-Man has recently become "Amazing" and his story yanked back to the beginning, pointlessly starting over with a different actor in the role and rebooting villains, girlfriends and storylines. And three years ago, after five X-Men films, the mutant characters were also whisked back to the past, with a collective origins story. At least that one felt fresh, X-Men: First Class turning out to be the best in the series.

But now Marvel and their Hollywood partners are at it again. And as the title suggests, they are again playing with time in order to prolong a cash cow.

I'm not suggesting that Days Of Future Past is a bad film; indeed, it is very good fun. Yet for all but the most devoted fan it will seem too self-referential, featuring too many head-scratching moments trying to remember exactly who did what with whom and when in the preceding films - with anyone who's ever had an X on his or her CV getting a walk-on. Any sequel or prequel (this is both) has to be self-contained to some degree; this one really pushes it.

It starts in a desolate future, ravaged by a war in which humans and mutants alike are victims of murderous machines called Sentinels. Xavier/Professor X (Patrick Stewart) and Eric/Magneto (Ian McKellen) have patched up their differences (the Prof seems to have returned from the dead, having been killed in Last Stand: head scratch No 1) and are looking for a way to save the world. Their idea: send Wolverine (Hugh Jackman) back to the past, to persuade their younger selves to bury the hatchet a few decades sooner and prevent the events that will lead to the creation of the Sentinels.

First Class took place in the 1960s, deriving humour, style and thematic context from the period, with the mutants being drawn into the Cuban Missile Crisis. Now Wolverine's consciousness is sent into his body in 1973, as Vietnam dominates the news. The first references are wonderfully comic, as our mutton-chopped hero wakes up on a waterbed, facing a lava lamp and next to a strange woman, with Roberta Flack's The First Time I Ever Saw Your Face playing in the background. It seems he's sleeping with a gangster's moll; once he's dispensed with the boss's henchmen he dons a leather jacket and hits the streets.

He finds the younger Xavier (James McAvoy) still smarting from the betrayals of First Class and drunk in his mansion; Eric (Michael Fassbender) has spent the last decade in a Pentagon cell, having apparently assassinated Kennedy; and Raven (Jennifer Lawrence), once torn between the two men, has become a one-woman mutant avenger. Wolverine needs to get Xavier on the wagon, Eric out of jail and Raven on message.

Bryan Singer, who helmed the first two X-Men films, returns to the franchise with his customary élan. And the first hour is particularly impressive, from the opening, future fight with the Sentinels to the tongue-in-cheek pastiche of the 1970s and a dazzlingly executed and amusing prison break, involving the speedy Quicksilver (Evan Peters). There's also a feast of retro-fashion starring Lawrence and a memorable head-to-head between the two Xaviers, McAvoy and Stewart. But though Fassbender is typically magnetic, the second half sees the screenwriters relying too much on Magneto's rebellious personality to keep their already strained plot in play.