Like all sci-fis, to begin with, you need to accept that it's possible to steal information from Johnny when he is asleep and dreaming. If you accept dream-burglary, you only need to push the boundary a little to agree it's possible to seed a new idea into Johnny's mind, while he dreams. If you are, now, on the same plane and convinced it is possible, I can begin explaining Inception.
Rules of the game:
If you die in a dream, you wake up.
If you die in a dream within a dream, (II layer of dreaming, mind you) you can be left in a limbo for a lifetime.
Leonardo Dicaprio is tasked, by the baddie, to plant a wicked Inception in Johnny's mind. To ensure the implant happens correctly, and deep enough, the team (LD's team) decides to sedate Johnny and take him to the second layer of dreaming. According to the plan, Johnny is tranquilised on a 10 hour flight from Paris to LA (in dream arithmetic, 10 seconds = 1 hour and, somehow, 10 hour flight = 1 week, but please don't ask me why or how?)
As luck would have it (or to lengthen the movie/story), Johnny is versed with dream-invasions and his auto-defence kicks in, which in the movie is depicted as security personnel shooting bullets at the speed of Concorde. One of these, freely raining, bullets hits Johnny, hence, before he dies, and to prevent him from getting into a limbo, the team decides to take him to the III layer of dream (hold your breath it's a dream, within a dream, within a dream) and succeed in seeding the villainous idea.
If that's not enough, and it hasn't got your head spinning by now, Leonardo Dicaprio (who is accompanying Johnny in the III layer of dream) decides to go in layer IV (dream, within a dream.........within a dream) to settle an age old agony/dream.
Statistics in US reveal that the movie has attracted repetitive viewing by audience, as a lot of viewers went into a limbo.
Why am I not surprised?
Recommendation: Watch at your own risk. You might have to watch it second time :-)