*** WARNING! SPOILERS! ***
I was worried that it would be like an Ironman 3, but I think every hero got enough air-time! I have been following each trailer and I don't think any of them gave away too much of the movie, which meant that we were still pleasantly surprised with the way things turned out.
The cube (called the Tesseract) is not only a source of unlimited energy, but it opens gateways between worlds. You see it in Captain America - it powered all of HYDRA's weapons and it was found by Howard Stark when they tried to retrieve Captain America after WWII. You also see the cube at the end of Thor, after the credits, when Nick Fury asks Dr Eric Selvig to start research into it. Avengers starts when Loki is sent back by an powerful alien race to retrieve the Tesseract and open the gateway between the two worlds so the Earth can be conquered.
The Tesseract is acting strangely and it turns out all those power fluctations it was having was that it was teleporting Loki to Earth. He wielded a staff which was forged with power from the Tesseract, which also allowed him to mind control Selvig and Hawkeye. S.H.I.E.L.D base gets decimated from an explosion from the unstable portal that the Tesseract was trying to open. So Loki takes his scientist and Hawkeye and goes off to try to construct the portal.
Black Widow was being "interrogated" and she looked like she was at some Russian crime boss's mercy. A phone call comes to one the crime boss's henchmen - the call is for her. "I'm working," she says to Coleson. And then with a sigh she fights them, Romanov style from her tied up position in the chair. Stark was busy with his new tower and having romantic times with Pepper. Captain America was still coming to terms with being a "Capsicle" for 70 years. Natasha was sent to bring in Bruce Banner. While everyone was looking for the cube and Loki, Loki popped up in Germany and the heroes go out to grab him.
Cap and Widow go to get Loki, and Ironman swoops in and they manage to subdue Loki and capture him. It seemed all too easy... like he wanted to be captured. Then as they were transporting Loki back, Thor appeared and snatched Loki. Ironman and Thor go after him and there was some muscle and ego flexing and battling. It was COOL.
A good play by the director, we got to see what we always like to see in a comic book to movie adaptation - hero vs hero battles. There was more of that later. Anyway, back on the big flying cruiser. Loki's captured, and the two brainiacs, Stark and Banner put their heads together to track gamma radiation, but they realise that S.H.I.E.L.D. isn't telling them everything. While the two of them try to hack into the S.H.I.E.L.D database, Cap takes a more direct route and goes into storage and finds all the weapons of mass destruction that were being made to battle against super hero types such as Thor and Hulk and other unknown alien forces. Natasha goes to question Loki about his plans and figures out that he is trying to get Banner to turn into the Hulk so that the ship can be destroyed from the inside. The heroes confront Fury and starts a round of bickering - all via Thor's influence. More ego flexing. Then Hawkeye blows up one engine and they are all shaken from their disagreement to a common purpose - save the flying boat!
The Hulk emerges and smashes up the ship. Loki escapes and ejects Thor from the ship in the cell made to imprison him and the Hulk. The ship freighter takes heavy damage - but they are saved in the nick of time. And Coleson, is killed by Loki - sad because he never got Cap to sign his collection of Captain America cards. The Hulk leaps out to attack a jet that was trying to lure him from the base ship. Fury uses Coleson's cards and hero ideology to goad the heroes to band together and save the world. And it turns out a good smack on the head is all that's needed to break Hawkeye out of his mind control. He and Natasha seem to have a bit of a "thing."
It's looking pretty bleak. The portal device is set up on the top of the new Stark building and activated.... and the aliens start invading. There's hundreds... versus Thor, Cap, Widow and Hawkeye and Ironman. They decide it's time to start playing as a team. Captain America assumes leader without too much quibble and directs Thor and Ironman to the skies, Hawkeye to be their eyes and he and Black Widow to be on the ground and try to save the civilians.
They seem to be holding back the invaders but they just keep coming... And Banner arrives, in his human form. And he switched into Hulk, and had SOME semblence of control over himself and went to take out the bigger ships, the size of blue whales. Natasha decides to jump onto a flyer to take the flight to the sky, and heads up to Stark tower. There, she finds Selvig, who had hit his head and now is free from mind control, and tells her the only way to disable the portal is to use Loki's staff. Loki was flying around on a flyer, but wa thrown off by an exploding arrow from Hawkeye and landed at Stark Tower, and met the Hulk. In what was the funniest scene in the movie, Loki is sneering at the Hulk about how insignificant he is compared to a king and God, and the Hulk picks him up like a rag doll and flings him back and forth on the floor multiple times. Oh I was killing myself laughing!
But by now, the governing council behind S.H.I.E.L.D. has decided that this disaster will not be averted by our heroes, but by a nuclear strike, and send a missile to Manhatten. Ironman intercepts it as Natasha is about to disable the device, and she stays her hand long enough for Stark to direct the missile through the portal to the big mother ship and blow it to smithereens.
Stark, stunned and unconscious, falls back through the collapsing vortex, and is snatched by Hulk who puts him on the ground, and his comrades contemplate his fallen body. The Hulk screams at him and he wakes up, asking if anyone kissed him... LOL.
And so, the Earth was saved. The Avengers go into hiding. The government calls them criminals. The world calls them heroes.
The movie was FANTASTIC. It was full of wit and humour and action and well orchestrated fight scenes and believable CGI. I would watch this movie again right now if I could - maybe another 5 times! And boy I can't wait till it comes out on Bluray :)