Cool quips and fleeting tenderness amid the superhero smackdowns from the Marvel Comic Avengers

Back to hold the ring for the second joint effort by the Marvel Comics Avengers, Joss Whedon does the job with typical panache in Avengers: Age of Ultron, but with so many characters jostling for attention, and so many sequel-friendly storylines being dangled, the danger is that all but the most dedicated of comic-book nerds will end up feeling befuddled rather than dazzled.

You certainly need your wits about you to cope with the film’s opening, which finds the gang dashing at full tilt under fire through a wintry East European forest. Their destination: a heavily protected castle where terrorist group Hydra is holding a mystical thingamajig with world-changing powers – the Staff of Loki, as any fanboy or girl will tell you.

Avengers: Age of Ultron - Robert Downey Jr as Tony Stark

Yet accomplishing the mission only triggers more mayhem. Robert Downey Jr’s wisecracking billionaire inventor Tony Stark, aka superhero Iron Man, wants to use a gem from the staff (one of six Infinity Stones, apparently) for a defence system that will protect the Earth against alien attack.  Instead, he succeeds in creating a giant sentient robot that believes humanity’s extinction is what is best for the planet.

The peacekeeping robot gone rogue is Ultron, voiced with enjoyably sly disdain by James Spader, and he soon has the Avengers running hither and yon around the globe before everyone ends up back where the action began in the fictitious Republic of Sokovia.

Along the way there are the ground-shaking dust-ups we’ve come to expect from the series – including a humongous clash in Johannesburg between two of the Avengers themselves, Downey’s Iron Man and Mark Ruffalo’s Hulk, put at odds by the mind games of Elizabeth Olsen’s Scarlet Witch, one of a pair of super-powered twins who have allied themselves with Ultron.

Avengers: Age of Ultron - Quicksilver (Aaron Taylor Johnson) Scarlet Witch (Elizabeth Olsen)

Her sibling is Aaron Taylor-Johnson’s fleet-footed Quicksilver – ‘He’s fast and she’s weird,’ is Shield agent Maria Hill’s verdict on the duo – a character so nippy he can apparently dash back and forth between two rival franchises, having appeared with a different backstory in last year’s X-Men: Days of Future Past, played by Evan Peters.

The battles have all the CGI-boosted heft a $250million budget can buy, but in the end there are only so many superhero smackdowns a body can take before it all becomes biff bam bore.

Indeed, the film is at its most entertaining when it finds the Avengers kicking back rather than kicking off, hanging around off duty and indulging in locker-room banter – as when members of the gang take turns trying to lift Thor’s mighty hammer Mjölnir.

Avengers: Age of Ultron - Black Widow, Scarlett Johansson

Scarlett Johansson’s slinky catsuit-wearing Black Widow doesn’t indulge in any of these displays of machismo, but she does get her share of Whedon’s one-liners – ‘I’m always picking up after you boys,’ she quips in the midst of one battle royal – and her tenderly romantic rapport with Ruffalo’s Hulk provides the film’s most affecting moments.

Whedon and his cast are at their best in scenes like these, but when the stunt doubles and CGI take over the relentless fights soon become exhausting. Then again, with two-dozen or so superhero movies from comic-book rivals Marvel and DC scheduled over the next four years (including this summer’s Ant Man and 2016’s Batman v Superman: Dawn of Justice), perhaps the aim is simply to pound us into submission.

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Certificate 12A. Runtime 141 mins. Director Joss Whedon.