Marvel fans will assemble — heh, heh — at theaters this weekend for the hotly anticipated sequel to 2012’s “The Avengers.”  So the big question on everyone’s lips, metallic or otherwise: Can “Age of Ultron” live up to its predecessor?

Well…not quite.

But that’s okay.

The film opens with the Avengers — Steve Rogers’s Captain America, Tony Stark’s Iron Man, Natasha Romanoff’s Black Widow, Bruce Banner’s Hulk, Clint Barton’s Hawkeye and Thor’s…uh…Thor — attempting to recover Loki’s scepter from a Hydra base in Europe.  There we’re introduced to the brother-sister pairing of super-fast Pietro (Aaron Taylor-Johnson) and telekinetic Wanda (Elizabeth Olsen), their powers the result of Hydra’s experiments.

After recovering the scepter, Tony (Robert Downey Jr.) and Bruce (Mark Ruffalo) use it to secretly develop an A.I. that will bolster a robotic defense system to supplant the Avengers.  Tony envisions a suit of armor around the world.  He calls it the Ultron program.  Once activated, Ultron (voiced by James Spader) decides to push his maker’s agenda to the nth degree:  the only way to save the planet is to eliminate the human race.

The first act of “Ultron” left me worried.  Despite the use of some CG-enhanced long takes, there’s very little sense of geography in the opening sequence.  The storytelling is more convoluted this time out.  (Seriously, my eyes went crossed writing those last couple paragraphs.)  We suffer through some pretty knotty exposition.  More than once, I found myself going Wait, who’s that?  Am I supposed to know this character?  Where’s so-and-so now?  Still, writer-director Joss Whedon weaves in some wonderful setup.  My favorite instance involves a pissing contest over Thor’s hammer.  Tony, Steve and Bruce try to lift it in an effort to prove themselves worthy.  The payoff to this is hugely satisfying and a great development at a critical juncture for our heroes.

The new additions to the Marvel Cinematic Universe are mostly strong.  This is the best Elizabeth Olsen has been since her acclaimed performance in “Martha Marcy May Marlene.”  Aaron Taylor-Johnson is sleepy as ever, but his screen time is short and lines of dialog even shorter.  Ultron is one of studio’s best villains, though the competition is admittedly light.  I enjoyed Spader’s dulcet tones as Ultron cracks wise – “I wanted to take this time to explain my evil plan” – though he does tend to prattle on.

And yes, he’s involved in another destroy-the-world plot.  Think about successful sequels like “Skyfall” and “The Empire Strikes Back.”  No fights for world domination, no planet-destroying battle stations.  Just personal struggles that make the stakes that much higher.

But “Ultron” isn’t without those intimate dramas.  There’s an attraction between Natasha and Bruce.  Though it’s pretty standard path – unfortunately, as Black Widow is the series’ most prominent female — the forbidden love angle works.  With the monster that rages inside him, Bruce is reluctant to let anyone get too close.  We also learn about Pietro and Wanda’s troubled past, which complicates their relationship to the Avengers.

Like many second chapters – I’m looking at you again, “Empire” – this one’s darker than the first.  Literally.  Ben Davis’s cinematography brings shadows into the frame, which are a nice change of pace from the brightly lit and generally flat “Avengers.”  Even the action feels heavier this time what with the percussive editing…though it can get tedious.  Crash!  Bang!  Repeat.  One sequence that benefits from the strategy is the fight between Hulk and a souped-up Iron Man.  To borrow from another comic book movie, this is what happens when an unstoppable force meets an immovable object.

The film isn’t all doom and gloom.  Whedon’s wit is as sharp as ever.  In one of the more comical scenes, Hawkeye embraces the absurdity that super hero movies traffic in: “We’re fighting a robot army, and I’ve got a bow and arrow.  None of this makes sense!”  Renner has a lot more to work with than he has in other Marvel outings.  Fans of Whedon’s “Buffy, the Vampire Slayer” might see parallels to Xander.  I certainly did. Hawkeye is a normal person amidst Gods (and some with God-like egos).  It affords him an opportunity to see things the others can’t.

Despite not being as strong as “The Avengers,” “Age of Ultron” is a very successful sequel.  It expands on the universe, introduces new characters, develops old ones and, per usual with Marvel, sets up things to come.