This is the latest big-budget addition to the Marvel Franchise. There are loads of characters from other movies. It seems a bit strange seeing them together on the screen; like this should happen in fan-fic and not the actual movie.
The plot is suitably ridiculous, a giant bad guy wants to collect six stones to give him ultimate power. The only oddity is that rather than using this power to destroy the whole universe he wants to destroy exactly half of it. To the film's credit this idea - bad guys collects stones of power - is introduced early on and you can then accept it and move on.
The heroes repeatedly defeat his minions but fail to stop him gathering more stones, until the inevitable final battle. With so many characters there's a few sub-plots going on. My favourite was Dr. Strange, the worst was Vision. I don't think I'd ever get used to calling someone Vision. The film is a bit more self-knowing than the other ones, and Spider-Man in particular makes light of them being heroes.
The only emotional connection is that the big bad guy's daughter is one of the good guys, so he is conflicted about killing her. This relationship is the core of the film, and expect will be what fixes the odd ending in the inevitable sequel.
With the huge amount of CGI the danger does not seem at all real, and the constant action scenes are colourful rather than really exciting. But there's enough constantly going on to keep me entertained, and I certainly felt I was getting value for money. Even watching late at night it never dragged, and I'll probably watch the next one too.