Starring Robert Downey Jr., Chris Evans, Scarlett Johansson • Directed by Joss Whedon • Age restriction: 10V • Releases 26 April

Since 2008, we've enjoyed a series of massive Marvel superhero films – Iron Man, Thor, Captain America and the rebooted The Incredible Hulk – all with an eye on this: the gargantuan climax of The Avengers.

And what a climax it is. Writer-and-director (and celebrity comic book nut) Joss Whedon directs this immense film, which is an expertly balanced product that should keep purists and casual fans alike thrilled from beginning to end.

Whedon has regularly proven that he can expertly manage ensemble casts, adroitly balance action and comedy and juggle complex mythologies – his work on cult classics such as Buffy the Vampire Slayer, Firefly and Serenity speaks to these strengths – and so his employment as a director and writer on The Avengers is a masterstroke. Whedon manages to keep the narrative moving at breakneck pace (and even though the film clocks in at around two hours and 20 minutes, it never feels overly long); balances deep moments of reflection with light-hearted and sarcastic quips – and still keeps a tight rein on his superstar ensemble cast.

And it’s quite a cast. Robert Downey Jr returns (and near steals the film) as the self-described "genius playboy philanthropist" Tony Stark; Chris Evans reprises his role as the solidly old-school and man-out-of-time Captain America; hunky Chris Hemsworth is back as what Iron Man describes as "Shakespeare in the Park" Asgardian Thor; while indie star Mark Ruffalo makes a turn as the third modern iteration of Bruce Banner and "the other guy" the Incredible Hulk. Rounding out this epic cast is Scarlett Johansson as the sexy, strong and dangerous superspy Black Widow; The Hurt Locker's Jeremy Renner as special agent and archer extraordinaire Hawkeye; Tom Hiddleston as the film's villain (and Thor's brother) Loki; while Samuel L Jackson finally has something decent to do in a Marvel film as SHIELD head Nick Fury.

When the Tesseract – the shiny cube you may remember from last year's Captain America – is stolen from under the nose of Nicky Fury, the Avengers are assembled to track it down. Its use as a source of unbelievable sustainable energy – and the fact that it can be used to open portals to another part of the universe – means that its recovery is a priority. SHIELD assembles a group of extraordinary individuals to capture Loki and recover the cube – and the Avengers are the world's last hope.

With so many characters at play, it would have been all too easy to lose some in the rush and focus on the fan favourites. But Whedon loyally and dazzlingly fleshes out each individual character, giving the stars time to shine and develop. Even if you've never seen a single Marvel movie prior to The Avengers, you'll be able to connect to these men and women. And that is the strength of this film –relatable, human characters with real, human struggles.

These superheroes are fundamentally flawed – in turn egotistic, doubtful and scared of their own power. It is the relationships between these heroes and our reactions to them – rather than the (still super-cool) action and special effects sequences– that make The Avengers a must-see. The dialogue sparkles, and this blockbuster is smart, funny and packed full of pop-culture references that will keep sci-fi and fantasy geeks swooning.

Some of the best moments come during the build-up to the final half-hour. Once the Avengers are rounded up, they're packed into an immensely cool airborne lair – part spaceship, part regular ship. And what happens when you trap a bunch of superheroes in an enclosed space? Well, as Bruce Banner describes – they're a "time bomb". The Avengers are pretty much the world's most dysfunctional family – and their infighting sets the stage for some of the biggest moments of character insight and development.

Robert Downey Jr pretty much steals the show in every scene he appears in,  and some of the funniest, most intelligent writing is allocated to him. His relationship with Bruce Banner provides deep, meaningful discussions about science, anger and human nature – and Mark Ruffalo brings a wry sense of self-deprecation to a room packed with huge egos. Whedon – who has created kick-ass female characters like Buffy and Firefly's Zoe and Yolanda/Saffron/Bridget – makes sure that Scarlett Johansson's Black Widow arrives with a bang, and is never just eye-candy.

And the sweetly wide-eyed SHIELD Agent Coulson – played by Clark Gregg – is a reflection of our own fandom. He has, we discover early on, collected a full-set of vintage Captain America trading cards – and wants nothing more than for the man-out-of-time himself to sign them. Coulson believes in heroes – just as we do – and it is his subtle presence that eventually becomes a spark of cohesion.

And the action and special effects? "Eye-popping 3D" gets thrown around far too often for my taste, but it's the best way to describe The Avengers. The film's climactic battle – where poor, oft-abused New York City gets destroyed again – is simply breathtaking. As the alien host makes its grand entrance – with incredibly designed "mother ships" that snake their way eerily through the famous skyline - the battle is compartmentalised but never feels separate as each of the Avengers splinters off to do his or her own thing.

Whedon edits the action down to easily digestible pieces, with cleverly used camera speeds to manipulate the action. The 3D is cleverly used throughout, so that you're still surprised by the additional depth in the last half-hour of the film. And I swear I could hear Whedon giggling in the background with the immense explosions and huge set-pieces chock-full of destruction.

The Avengers is simply exhilarating – there's no other word for it. Buy in to the hype. It's a must-see.