There was an idea, started ten years ago with a scene that most audience members at first left too quickly to see, played after the end credits of a then B-list superhero’s film debut starring a barely insurable actor still in career recovery.

"Iron Man," however, quickly became a phenomenon, and one decade, 18 films and a box office gross just under the GDP of Djibouti later, Marvel has arrived at the promise of that first post-credits scene, the culmination of its forever blockbuster-altering work, with the massively epic, mostly entertaining "Avengers: Infinity War," the largest superhero gathering ever assembled on the big screen. It certainly feels like it – with all of the exhilarating payoffs, as well as a few of the exhausting problems, you would expect from cramming an entire universe, cinematic and literal, into a single sitting.  

Restless from sitting on his space throne, doing nothing while his anonymous lackeys (not even Ronan the Accuser remembers Ronan the Accuser) come back empty handed, Thanos (voiced by Josh Brolin) finally decides to leave the house and hunt on his own infinity stones, six glowing gems that each control a different cosmic power and, when joined together on his special gauntlet, could wipe out half of, well, every living thing in existence.

Most beings would rather he didn’t, so superheroes from across the MCU unite to stop his quest for the glitziest glove since Michael Jackson. Thor (Chris Hemsworth) teams up with the Guardians of the Galaxy to hunt down the gems previously in their possession, but now scattered across the solar system, while Iron Man (Robert Downey Jr.), Doctor Strange (Benedict Cumberbatch) and Spider-Man (Tom Holland) take the fight back to Thanos’s home planet after catching an accidental ride on a spaceship. Meanwhile, Black Panther (Chadwick Boseman) and an impressively bearded Captain America (Chris Evans) protect the infinity stone-powered Vision (Paul Bettany) from Thanos and his army in Wakanda. And apparently somewhere Hawkeye is drinking a mimosa and enjoying retirement.

It’s hard to believe there was a time when "The Avengers" seemed like a risky idea, that gathering all these characters on screen together into a coherent story would be a certain disaster. Joss Whedon’s 2012 monster hit, however, pulled it off with breezy ease, and since then, Marvel’s turned the comic book crossover into a science (one that keeps exploding in other attempted universes’ faces), introducing and fleshing out characters in their own films before ably bouncing the colorful personalities off one another in collaborations big and small.

"Infinity War," obviously, falls on the former side and thankfully is equal to the sum of its XXL-sized cast, smashing together entertainingly banter-happy combinations such as the clashing egos of Stark and Strange or the cosmic goofballs Thor and the Guardians of the Galaxy (Dave Bautista’s overly literal Drax is yet again a scene stealer). Between the careful casting and character work of the past installments and the latest's script from Marvel regulars Christopher Markus and Stephen McFeely, "Infinity War" makes a lot with a little – from a brief reunion between Bruce Banner and Black Widow rich with melancholy to Steve Rogers’ hilariously perfect introduction to Groot. The shorthand helps the movie nimbly leap from witty levity to weighty drama – easily the heaviest and most grim delivered so far.

The most essential character to "Infinity War," however, is the guy on the other side of the fight: Thanos, the big bad teased for so long and so lurchingly that he’d become less on par with an iconic villain like The Joker and more just a joke. Combine managerial skills barely sufficient to run a McDonald’s much less a universe-spanning genocide with worrisome CG and this former oversized purple puma seemed destined to join Steppenwolf from "Justice League" as villains unworthy of their massive stage.

But Thanos easily surpasses his lukewarm build-up and quickly becomes a more than suitable foe for such a massive superhero tag team match. Sure, he still looks like an oxygen-deprived thumb whose chin took a tough trip over a pasta roller, but the script – with an assist from Brolin’s shockingly effective, emotive motion capture work – gives him an actual philosophy and purpose for his villainy, with even a surprise dash of vulnerability thanks to his painfully damaged relationship with his step-daughter Gamora (Zoe Saldana). Like the best villains (and unlike most Marvel baddies) he’s a fully developed, interesting character, not just a generic boss fight – though he’s also a legitimate threat throughout the movie’s action sequences.

Its many, many, many action sequences, that is. It’s called "Infinity War," after all, not "Brief Quarrel," and Marvel’s latest earns the name with multiple epic set pieces. Directors Joe and Anthony Russo ("The Winter Soldier," "Civil War") stretch their visual muscles, ditching the drably grey look of their last two efforts for deeper shadows and brighter colors, making a blockbuster that pops like a comic book movie should (and like the last several Marvel movies have).

Brought to an intergalactic scale, the director duo’s action is still tight, delivering fistpump-worthy action beats and having fun finding new ways to play with our heroes’ powers – so much so that you wish the movie pushed them even further. More confounding reality-bending and physics-breaking, please, and less of the typical punching of CGI hordes of faceless Venoms.

It’s those moments that stick out in the barrage that is "Infinity War." Between more than two dozen superheroes, six infinity stones, a few other plot devices to pick up and a fairly constant march of action scenes, "Avengers" feels like five different movies all happening at once and moving full speed ahead at all times. Just ten minutes away from Thor and Rocket requires your brain to refresh itself: Wait, why are we on this weird metallic planet talking to a giant Peter Dinklage? And hold on, did Thanos throw a moon at somebody? That feels like it should be a big deal, something I'm still gathering my jaw from the sticky cinema floor as a result of, not already forgetting in order to move on to the next scene and the next cool but fleeting moment. It’s hard to discern any real sense of awe or wonder from the sound of all the gears and machinery moving "Infinity War" constantly forward, an occasionally repetitive plot doing a lot of work to make up for Thanos's last 18 movies of failure. 

But as previously noted, Marvel does have its formula down to a science – for better and for worse, mostly for the better in this case. The now signature quippy popcorn ease of the interactions mixed with buoyant action certainly help cut through the inevitable clutter. And when the movie does take the time to come up for air, the moments really echo – such as Thor coming to terms with everything he’s lost across all these films so far or the early eerie panic of Thanos's donut-shaped spaceship arriving in New York. It's not Marvel's best movie – heck, it's not even the best movie Marvel's released this year – but its emotional punches hit the hardest. 

That's especially the case in the final act, which is easily the most powerful, devastating work Marvel’s brought to the big screen so far, playing with one’s comfortable expectations from a superhero movie and dropping down the weight one would expect from the franchise's apex point. It's also easily going to be the most controversial part of "Infinity War" moving forward. Without spoiling anything, some will call it a cliffhanger, but it actually nicely wraps up the battle for the infinity stones story while pointing out the path for part two in 2019. (Perhaps nicely isn’t the right word, though.)

Considering Marvel paid off 10 years of build-up with "Infinity War," one would hope it can now pay off just one year of waiting for the true finale. 

"Avengers: Infinity War": *** out of ****