Following the events of Avengers: Assemble and Captain America: The Winter Soldier, the superhero team of Avengers spring into action to foil a Hydra plot. In the aftermath of this battle, Iron Man discovers the dormant remnants of a peacekeeping AI programme. When he tries to unlock it, he unwittingly opens a Pandora’s Box of anything but peace.

The best decision Marvel have made when it comes to the Avengers was to appoint Joss Whedon at the helm. He had already shown what a dab hand he is at mixing exciting, original SF with superior character interplay with the exceptional Firefly. It elevated Avengers Assemble, kickstarting the series in all the right ways, and it’s this same touch that raises Age of Ultron above the ordinary.

Whedon is fond of balancing action with emotion and humour in almost equal measures. Certainly, Age of Ultron‘s rougher edges are smoothed by plenty of joshing between the central characters. Quips, winks and one-liners are every bit as rife as the explosions. The director manages to keep these just the right side of smug, though it does occasionally flirt with straying into “backstage with the Avengers cast” territory.

There’s a fair amount of heft in the emotional scenes too. Jeremy Renner’s Hawkeye, in particular benefits from this. There was a worry that he would go the way of Cyclops in the X-Men movies and end up relegated to cannon fodder status while the all-star leads steal the best moments. But here he’s welcomely fleshed out more than he was in Assemble and given his own “hero” moments.

So too Mark Ruffalo’s Hulk and Scarlett Johansson’s Black Widow, as each questions where they fit in the good guy business. They form the focus point for Age of Ultron‘s central theme: conquering inner demons. One section is particularly effective in exploring this; it’s trippy stuff, drawing on some unexpected sources of cinematic inspiration. Here it veers into shades of horror, throwing nods to the likes of Jacob’s Ladder and Ghost Ship.

You might have guessed from this that the Tesseract is back in play. It is the icy blue vein running through the body of this sequel, linking the movie unmistakably to its predecessor. In amongst the noise and clamber, meanders this stream pointing to the horizon, hinting at what lies beyond this particular chapter of the Marvel universe.

So, in amongst all the emotional drama, comedy and subtext, there’s action, right? Rest assured, straight from the off, there’s plenty of that. All the heavyweights get to show off their fighting chops. Thor hammers, Iron Man brings the metal, Captain America plays extreme frisbee and Hulk … um, well … smashes. This is a superhero movie after all. There’s meant to be a feast of spectacle and each of them grabs just enough of the limelight to keep it balanced.

That said, there are a host of new characters introduced. It’s here that the movie gets a little bloated, especially in Aaron Taylor-Johnson’s Quicksilver and Elizabeth Olsen’s Scarlet Witch, who come across as escapees from the mutant world of X-Men.

And still the sceptre – sorry, spectre – of Avengers Assemble looms large. In this, the James Spader-voiced Ultron is no match for Tom Hiddleston’s gloriously mischievous Loki. The main problem is the actual character: Artificial Intelligence come to realise that its intellect is greater than its masters’ and thus rises up to threaten Earth? Yes, we have been here before. Perhaps Ultron himself says it best: “I will take your creations, your inventions, and I will turn them against you”.

It’s a shame also that several of the action set-pieces end up like hyperactive children, desperately trying to show they’re as good as their celebrated parent. Predictably, this leads to a mixed bag of goodies, with some better than others. At least the film pulls out all the stops to keep it fresh, though. There’s some nice use of slo-mo, framing several memorable scenes as if they were suspended within a snow-globe, as the makers continue to blur the lines between comic book panel and film.

Well, what do we make of it all then? It would have taken some effort to match, never mind surpass, Assemble; in that regard, Age of Ultron falls short and there is more than a hint of a comedown from those giddy heights. But it would be churlish to judge it too harshly by its own lofty standards. This is still excellent entertainment, writ through with plenty of wit and snappy dialogue. The actors have grown nicely into their roles and their confidence keeps it soaring. It never gets tedious and largely manages to balance its various elements.

And now, we await the Infinity War.