I *love* this movie. The plot is not insanely complicated, really, in the broad outlines.
Technology exists that allows people to share dreams. I'm assuming that it had some theraputic applications which it was originally invented for. Cobb (Leonardo DiCaprio) and his team use it for
So a man, Mr. Saito (Ken Watanabe), hires this team. Not to steal ideas, but to implant one. He wants to destroy a rival company by having the son, played by Cillian Murphy, break up the empire after he inherits it from his dying father. Of course, planting an idea is infinitely more difficult that stealing one, because the persons brain can always track down the thought to it's origins, and subconsiously, if nothing else, recognize that it originated from somewhere else.
They create the world, drop Robert Fischer (the son) into it without his knowledge, and take him down through three levels of dreams, slipping into his brain in insane ways, until he thinks that it's his idea. That his father really wanted him to destroy his life's work, so that Robert can be his own man.
It's not a complicated plot (though of course there's much, much more to it, and I'm leaving lots out despite my not spoiler safe policy, because you really need to *see* this movie), but it is so insanely well written and acted it's not even funny. It's mind bending at it's finest, and you leave the movie having to almost pick an ending. Is Cobb out of the dream? Has everything worked out for the best? Is his happiness real? Or is the top still spinning?
ALSO: Joseph Gordon-Levitt. My dear boy, I forgive you for G.I. Joe. If I acted, I totally would be in a campy movie of crapitude if I got the chance to play a kick ass villain too. Other than that, you're doing *awesome*.
No, the section below is fairly spoilery. You've been warned.
So, Cobb basically plays god here, with the implantation of an idea in Fischer's mind. The other people in his crew, they at first insist that inception can't be done. You can't introduce a foreign thought into someones mind and have them come out believing that it was their idea. It's too complicated, too messy. The mind eventually tracks it back and realizes that it popped in there, fully formed from elsewhere, and rejects it, because it cannot find a natural evolution for it.
The idea that Saito wants introduced is that Fischer wants to break up his fathers empire. It's complicated, because of all the political, personal, and financial ties. So they have to come up with an *extremely* simple concept to slide in there that will net them the same result - the destruction of the Fischer energy empire.
Eames, who is a forger, explains that emotions are the way to go. They're the most basic, and if an idea can be thought to be based on the persons emotions, they'll take it in as their own. In theory, of course, because no one *does* this, because everyone knows it can't be done.
So they slide into Fischer's brain, take him down into three dreams. A dream within a dream, within a dream. And on each level they work their way deeper into his subconscious. The first level they make him believe that he's awake, and he and his godfather (who is the only male role model he has a good relationship with) have been kidnapped for the combination to a safe in his father's office. The father is already dead at this point, by the way. Eames is 'playing' the godfather, so that he can explain what is in this imaginary safe, which is a second will allowing Fischer to dissolve his father's empire, saying that Fischer Sr. wanted Fischer Jr. to be his own man, and to build something bigger and better. Then, from that, they slide him into a second dream, where they lay the suspicion that his godfather is actually in on the kidnapping, and is manipulating him. So, in order to find out what his godfather is up to, Fischer (thinking that Cobb and co. are parts of *his* subconscious trying to defend him from the invasion of his godfather's goons), goes into the third dream, thinking that he's breaking into his godfather's mind to get at the truth there, when, in fact, he is breaking into his own deepest subconscious, and taking Cobb and co. with him, so they can see if their plan to reconcile him with his father and make him believe that the breaking up of the empire is what the father really wanted for his son is working.
Now, here's the playing god *recklessly* part. The other people, they believe that inception can be done because Cobb tells them that he's done it before. The important factor that he leaves out is that he did it to his wife, to rescue them from a dream state that they couldn't escape. He planted the thought that her world was not real, so that she would 'kill' herself with him in the dream, thus kicking them back into consciousness. And it worked. However, what he didn't realize was that thoughts are like viruses. They take over, and reshape the person. Become their entire being. They were back in the real world, but Molly (Cobb's wife), was consumed with the thought, with the belief, that her world was not real. And, trying to save herself from this 'dream', she killed herself.
The first time was an accident. Cobb had no way of knowing the ripple effect that would occur. But, when he agreed to do it to Fischer, he knew perfectly well that there is no predicting, and no controlling, what happens once the thought has been accepted. Certainly, it *seems* harmless enough for one really, really rich guy to destroy his fathers life work. But, it's not just that. At the last dream, Fischer's father tells him that he wants him to build his own 'empire', to surpass the father. How do you know that Fischer, Jr. doesn't surpass his father in really horrible and destructive ways? Sure, Fischer seemed like a sensitive, kind enough sort, but (to follow Godwin's Law) Hitler was an artist before he rose to power.
Cobb is the only one who could have had any idea that this might run out of control, and that there's nothing they could do to stop it or control it, but he didn't care. It was all about getting home and being with his children again. It absolutely did not matter to Cobb that he could be unleashing a monster on the world, so long as he got what he wanted.