Avengers: Age of Ultron
Three stars (out of four)
PG-13 (intense sequences of sci-fi action, violence and destruction; suggestive comments)
141 minutes

Thor, Captain America and Iron Man may be the flashiest, most popular Avengers but they’re also, arguably, the least compelling members of Marvel’s superhero collective.

Thor (Chris Hemsworth) has fabulous hair, a big hammer and wrestles with Shakespearean family drama.

Captain America (Chris Evans) is decent and square and also kinda sad that everyone he ever knew and loved is now dead.

Iron Man, aka Tony Stark, is Steve Jobs with better hair, nicer clothes, more charm and an obsession with technology that is both an asset and an Achilles heel.

These guys are great and all, but they’ve each starred in at least two solo movies apiece. By now, we know pretty much everything there is to know about them.

So it’s an unexpected pleasure that “Avengers 2: Age of Ultron” devotes its attention to characters who spent a lot of time lingering in the background in 2002’s “Avengers.”

At last, we discover everything we’ve ever wanted to know about Hawkeye (Jeremy Renner), the stoic archer who skulked through the “Avengers” in a Loki-induced trance.

We also find out just what is going on between him and lethal assassin Black Widow (Scarlett Johansson), who piqued our curiosity with the tiny gold arrow she wore around her neck in “Captain America: The Winter Soldier.”

The relationship isn’t quite what we expected and that’s half the fun of “Age of Ultron.” The movie brims with enjoyable little surprises, from cameo appearances by minor characters to clever winks to previous Marvel outings.

The Hulk finds romance. Black Widow gets to be vulnerable as well as spectacularly lethal. Maria Hill actually cracks a few jokes (only natural considering she’s played by funny-girl Cobie Smulders).

Jarvis the computer, who has always been one of Iron Man’s most sharply sketched personalities, thanks to Paul Bettany’s tart voice work, undergoes a delightful evolution.

If the Marvel movie franchise has become an almost impossibly tangled web, director Joss Whedon is a nimble spider, spinning off dozens of new plot threads, wrapping up neat, little moments for a vast ensemble of characters, deftly interweaving CGI spectacle and satisfying emotion. This is movie-making on an unprecedented, gargantuan scale. It’s no wonder the guy needs a break.

When it comes to theme, “Age of Ultron” doesn’t break much new ground. United, the Avengers stand. Divided … well, not so much.

The glories of the team’s combined might are illustrated in a prologue that sees the superheroes working in perfect harmony as they ambush a Hydra base in the snowy woods of the fictional Eastern European nation of Sokovia.

Our band of heroes emerge victorious with a new toy for Stark (Robert Downey Jr.) to play with, containing one of the infamous infinity stones that Marvel villains are always after. They also acquire a pair of new enemies, eerily gifted Sokovian twins played by Emily Olsen and Aaron Taylor-Johnson.

When Tony starts poking into the infinity stone’s properties, back at the shiny S.H.I.E.L.D lab — or at least the corrupted organization formerly known as S.H.I.E.L.D — he discovers alien technology perfectly suited to realizing his pet project: an artificial intelligence program powerful enough to enforce world peace.

Without bothering to consult the other Avengers, Stark talks the skeptical Bruce Banner (Mark Ruffalo) into helping him create the super program known as Ultron. Apparently, these guys have never seen “The Terminator,” because the being they spawn is a malevolent, red-eyed robot who misinterprets his mission to disastrous effect.

Once again, the Avengers begin to doubt each other, especially as Olsen’s Scarlet Witch — a welcome new female presence in the testosterone-filled Marvel landscape — unleashes her witchy powers of mind control upon them, causing them to relive painful pasts and envision future fears.

Leading the clash of consciences are Stark and The Cap, whose dueling philosophies on power and peace put them deeply at odds. (Could this be the beginning of a certain Civil War?)

At this point, the Marvel universe has become so complicated — spanning multiple galaxies, planets, dimensions and eras — that plot almost ceases to be relevant.

While I enjoyed nearly every minute of “Age of Ultron,” I felt at times as if my grasp on the whole thing was slipping. Who could say exactly what was happening at any given moment?

I don’t think it’s just me and my sometimes foggy, sleep-deprived brain, either. My theory is that, at this point, only the Marvel script supervisors know precisely what is going on.

Still, there’s a familiarity that anchors us.

Elements of “Beauty and the Beast” can be found in the movie’s unlikely central romance, even if the coupling comes out of left field.

There are shades of the “Frankenstein” myth in Ultron, who proves to be one of Marvel’s more fascinating baddies, thanks to James Spader’s acerbic vocalization.

As lofty, and perhaps unachievable, as its ambitions are, it isn’t the money-shot action sequences that ground “Age of Ultron.”

The film is at its headiest and most thrilling when it puts the mayhem on pause for the sake of intimate interactions between its god-like heroes — trading war stories at a party, licking their wounds after retreating to a remote farmhouse.

The Avengers are most endearing when they are most human.