Phew. Excuse me for a minute while I let my brain settle down. This movie is full of explosions, but it is also a thinky movie. This is a very thinky movie.
In fact, it tends to think out loud. New villain Ultron (voiced by James Spader) spends
so much time monologuing that it's a wonder it takes the Avengers so long to defeat him (at least fifteen minutes longer than it should have.)
Ultron is the brainchild of Tony Stark (Robert Downey Jr.), genius, playboy, billionaire, sudden techno-philanthropist-Utopian. Tony's been spending his free time creating an artificial intelligence which will establish lasting peace. Oddly enough, he's keeping the plan to himself, resisting the urge to brag. When he does tell Bruce Banner (Mark Ruffalo), he stresses the need to conceal "Ultron" from the other Avengers. They'd merely argue about it and by then it would be too late to succeed.
Ultron is the end-game, he explains. This program will put the Avengers out of a job, and create Utopian harmony. Inevitably, something goes wrong, and in trying to create a perfect world, Tony creates the very thing which will destroy it. Ultron escapes into the internet, respawning wherever need be, to enforce his new order and attempt to exterminate the Avengers.