As everybody says, this is really an Avengers movie, though Captain America is the protagonist, and like the previous Captain America film it touches intelligently on some serious subjects, including unintended civilian deaths and whether dangerously powerful superheroes should act as free individuals or be subject to some sort of government control. Disagreement about the latter leads to a split in the Avengers, with Steve Rogers (Captain America) on one side and Tony Stark (Iron Man) on the other.
There are also a lot of funny bits, and Ant-Man and Spider-Man show up as well.
This version of Spider-Man is back to being a high school student who lives with his Aunt May, but not the very elderly Aunt May of the comics. This Aunt May is played by Marisa Tomei, looking as attractive as I’ve ever seen her, which is attractive indeed. When Tony Stark (Robert Downey Jr) meets her his reaction is pretty much the same as mine: “You know, I'm having a hard time believing she's someone's aunt.” He can’t shut up about it.
I enjoyed it, but if you haven't seen at least some of the films leading up to this one you might be lost.