The story this time around is of course a new supervillain for the Avengers to face, this one intent on world domination like all the rest of them, but played by James Spader in a motion capture performance that makes use of his signature fantastically slimy voice. He's Ultron, the ultimate artificial intelligence robot, a creation sprung from the mind of Tony Stark (Robert Downey Jr.) who wants to use him as a force for good, with the idea of a world peacekeeping mission in mind. But things get out of hand and Ultron quickly turns evil- I'm not sure exactly how that happened, except that all robots in movies must eventually become smarter than their inventors and desire to take control- this one wastes no time in getting to that point (it takes about ten seconds actually). So after an opening sequence that sets the Avengers on some kind of avenging mission that makes them look like video game characters bouncing around a forest, they celebrate their victory at an after party where various heroes mingle and wisecrack and flirt (this is where we find out that Scarlett Johansson's Black Widow apparently has a thing for Mark Ruffalo's Hulk- not sure why or when that started, but it's bound to never be important or brought up again, so don't bother to invest in it too hard). Scenes like this are always the best part of these ensemble movies, simply because watching the guys hang out is kind of a basic pleasure we don't get enough of, due to the constant barrage of action slated to occur after no less than 12 minutes of lagtime has passed (chalk that up to studio notes I'm sure- me, I'd prefer an entire movie of the Avengers hanging out and partying).
Ultron crashes the festivities soon enough though and goes about his quest to take over the world, bringing in a couple of new guys, Russian twins Quicksilver and Scarlet Witch to assist him in his dastardly deeds. The twins (played by Aaron Taylor-Johnson and Elizabeth Olsen) aren't inherently uninteresting, but the movie is so crowded with characters that their backstory is glossed over, and just as soon as they're set to bring down the Avengers, they've suddenly changed their minds and are teaming up with them instead. No sooner does that happen than the movie introduces yet another hero, the Vision, who's the enhanced version of Paul Bettany's Jarvis, and by that point the movie becomes overstuffed with the sheer amount of costumed heroes on the screen and it's overwhelming to keep up with all of them. Yet Joss Whedon does what he can, deftly balancing sequences that veer from chit-chat to action and back again, handling it all in as swift a manner as possible without ever getting too bogged down in the weeds with technical jargon or know how. The movie tosses out plenty of Easter eggs to comics fans about events coming in future films (at one point Thor simply disappears to go off and find the "infinity stones," which won't be in play until at least the next Avengers movie), but ultimately everything happens exactly how you're expecting it to- heroes save the world, bad guy is defeated, the gang splits up and we're on to the next thing. It's spectacle that many people love, and that I myself kind of passively enjoyed- I just can't bother to care very much.