When Thor's evil brother Loki comes to Earth, he steals an energy cube called a Tesseract to open a portal to a different dimension so another alien race can invade. He also takes control of several people to help him in this endeavor, including Hawkeye.
When the Avengers ‚Äî including Thor, Ironman, The Hulk and Captain America, Black Widow ‚Äî are brought together to help figure out a way to save Earth, they start fighting, arguing and goading each other. Each is a superhero and has worked alone successfully (or in The Hulk's case, so he doesn't hurt others). Working together proves to be much more difficult than expected, especially when Loki does his best to split them up.
Once they figure out how to use their strengths to benefit their mission to save Earth along with cooperating, this group of superheroes just might be able to thwart Loki's plans.
And just one more note: The only Marvel comics show I saw before this was "Captain America." It would be helpful to know at least some of the background, but if you haven't seen any of the other shows, you wouldn't be completely lost.
There is a lot of violence and the PG-13 rating it definitely warranted.