This is a pretty long film, and also fairly linear, but surprisingly enjoyable. Short version of the plot: Loki, the Asgard God, is after The Tesseract, a mcguffin that has ended up in SHIELD's hands on earth, so he decides to turn up and claim it. Earth, that is. Nick Fury assembles the Avengers, and punching happens.
The script's a work of art. The lead characters - Samuel L. Jackson's Nick Fury, Robert Downey Jr's Tony Stark (Iron Man), Chris Hemsworth's Thor, Chris Evan's Steve Rogers (Captain America), Scarlett Johansson's Natasha Romanoff (Black Widow), Mark Ruffalo's Bruce Banner (The Hulk), Jeremy Renner's Clint Barton (Hawkeye) and Tom Hiddleston's Loki - are juggled sufficiently well to both get things to do and rub each other up the wrong way, as befits their characters. Plus, there's Clark Gregg's Agent Coulsen, Gwyneth Paltrow's Pepper Potts and Colbie Smulders' Agent Hill all getting a word in as secondary characters.
Favorite bit: Rogers, Stark and Banner all reflecting on different things they find baffling/wrong with the situation, whether they're being manipulated, and by whom.
There's some very funny bits in here, too. And for one of them, the audience applauded. I honestly can't remember the last time a cinema audience did that. Hats off to Joss Whedon and Zak Penn.
Now, can we please have a Black Widow movie? Preferably two, instead of the inevitable sequels to either Captain America or Thor.