The interesting thing about the first Avengers film, the most successful Marvel movie of all time, is that director and writer Joss Whedon considers it a failure.

Taking in an estimated $1.5 billion at the global box office, Avengers was the third-biggest-grossing film in cinematic history, behind Titanic and Avatar. But when Whedon looks at it, all he sees is "flaw, flaw, flaw, compromise, laziness, mistake," the filmmaker told Vulture.com.  

The reason Whedon, the showrunner behind fan favourites such as Buffy the Vampire Slayer and Firefly, committed to make the second Avengers film, he says, was to address his mistakes.

For his troubles, Whedon says, the experience nearly killed him, and it's easy to see why. 

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The Avengers: Age of Ultron is not about forming the superhero team, but finding a reason to justify its existence while simultaneously tying up the loose ends of three film franchises and their ancillary TV shows. Whew. (Disney/Marvel/Associated Press)

The first film was a delicate balancing act, but there was a firm template anchored by an origin story about a team of heroes uniting to stop pesky God Loki. By comparison, Avengers: Age of Ultron has a much more difficult mission.

It's not about forming the team, but finding a reason to justify its existence while simultaneously tying up the loose ends of three film franchises and their ancillary TV shows.

Darker subtext

The highly anticipated sequel, which opens in theatres on Friday, starts at a sprint pace, as if Whedon is eager to apply the lessons he's learned. The action is tight and dynamic as the team springs into action and attacks a base of the world-wide terrorist organization, Hydra.

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The Avengers get some new, hostile company in Age of Ultron in a pair of mysterious and enhanced twins, played by Elizabeth Olsen, right, and Aaron Taylor-Johnson. (Jay Maidment/Disney/Marvel/AP Photo)

​But there's a darker subtext in the air. When Robert Downey Jr.'s Iron Man summons an army of robot drones called the Iron Legion, the crowd pelts them with garbage as the camera lingers on Banksy-style graffiti where Iron Man's head is replaced by dollar signs.  

Building on that sense of resentment, Captain America (Chris Evans) and friends soon meet two new enhanced opponents: Quicksilver, played by Aaron Taylor-Johnson, and Scarlet Witch, played by Elizabeth Olsen.

The orphans, who speak with a Slavic snarl, are looking to teach Tony Stark a lesson. They were raised in the fictional eastern European country of Sokovia, "a place that's nowhere special on the way to everywhere special," as Maria Hill describes it.

It's a throwaway line, but also an example of why Whedon's deft dialogue is his super power.

Best Hulk ever

While Avengers: Age of Ultron threatens to sag under the strain of the various strands of the story, it's the little moments that make the Avengers seem real.  

In a movie with 10 major characters, Whedon almost manages to give everyone a moment, and if you're a fan of ol' Greenjeans (that's the Hulk for non-comic nerds) you are in for a treat.

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Mark Ruffalo plays The Hulk in a scene from Avengers: Age of Ultron. (Disney/Marvel)

Keep your eyes peeled during the party scene at Avengers headquarters for a film noir throwback.

Mark Ruffalo's Bruce Banner sidles up to Scarlett Johansson's Natasha Romanoff at the bar with the old "What's a nice girl like you doing in a dump like this?" line.

"Fella did me wrong," she replies, and we're off to the races with a wonderful variation on a classic Whedon theme: the Beauty and the Geek.   

The Hulk and Black Widow are just two tragic heroes trying to find a little solace in the gamma-charged heart of this film, and Ruffalo has created the best version of the Hulk we've seen.

Smug super villain 

While we're on the topic of characterization, Marvel fans should get ready for a different type of super-villain: Ultron.

Voiced by James Spader, the sinister robot with delusions of grandeur is smug, sarcastic and practically jocular. 

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Ultron, a product of Tony Stark’s artificial intelligence program run amok, makes for a different type of super-villain with his smug sarcasm. (Disney/Marvel)

And while an android this amusing strains believability, the result is a hoot and a major improvement on dull world-destroying despots. (I'm looking at you, Ronan.)

While Hulk and Ultron shine in this sequel, not everything is perfect. Whedon's first cut of Avengers: The Age of Ultron was an epic 3 ½ hours. The final version runs two hours and 22 minutes. That's over an hour of footage gone, and it shows. 

Been there, done that

There is also a niggling sense of the familiar to this tale. For the plethora of new characters and a more global setting (South Korea, Wakanda and more) the finale finds the Avengers once again buzzing around trying to protect a city from a faceless horde.  

Sure, Ultron's robot army is an upgrade over the alien Chitauri, but there is still a feeling of been there, done that.

Zoom out to the bird's-eye view and you'll recognize another pattern: Massive action scenes followed by casual moments where we get to see Avengers relating on a more human level.

We see them joking about trying to lift Thor's hammer at a party, and sitting down for a home-cooked meal in another scene. As much as I revel in the action — like Iron Man and Hulk going at it in a stunning slugfest — it's still the quiet moments, like Thor sheepishly stepping on a toy, that are most endearing.

The key to enjoying Avengers: Age of Ultron is just accepting what it is. This is not Whedon's The Empire Strikes Back — a film that had the courage to leave the audience in a dark place, desperate for more. 

If anything, this is Whedon's Return of the Jedi.

​Avengers: Age of Ultron is his swan song, a chance to play the hits one more time, improve on the formula and leave the pieces on the table for the next iteration.