Saito offers Cobb a way out, to work for him on an extremely important project — to enact an "inception," placing an idea in the mind of his top competitor Robert Fischer, Jr. (Cillian Murphy) to get him to break up his dying father's (Pete Postlethwaite) company that has a monopoly that is squeezing Saito out of the market. While Arthur doesn't think it can be done (since they have previously only done an "extraction" of information), Cobb agrees to take on the job of inception because he says, "I've done it before."
The secret of how and why Cobb knows inception will work is eventually revealed, but it is so crucial to the plot that I will not spoil it here; however, this knowledge haunts Cobb as does the image of his dead wife Mal (a powerful performance by Marion Cotillard). For those of you who might say this sounds familiar, you'd be right. DiCaprio's character Teddy Daniels was haunted by dead wife Dolores (Michelle Williams) in last year's Shutter Island. The similarities are a bit peculiar, but they do diverge enough not to make it a distraction to the viewer.
The most intriguing thing about Inception is the way Nolan has depicted the dream world that he has created. Within this dreamscape there are multiple levels, dreams within dreams, and like a play within a play, they open new dramatic doors and the possibilities become endless. Depending on who is dreaming, we can sink down two, three, or even four levels into a maddening psychological miasma where drowning is always an option.
On the first level of dreaming, a person can be killed and thus forced to end the dream, waking him or her up. As the stakes get higher in the game Cobb plays with Fischer (including taking powerful narcotics to keep the dreamers asleep), the easy out of killing someone is lost. A person who "dies" in a dream falls into a deeper chasm or "limbo" from which it can take a long time (decades perhaps) to escape, reminding one of what Carson McCullers called "the yearless region of dreams."
There are many elements involved in getting the idea to break up the company into Fischer's mind. Once the idea is successfully implanted, he will wake up without realizing what has happened but thinking the concept is an awareness of his own making. While all of this is very complicated, Nolan has handled the multiple levels in a dynamic and powerfully visual way that keeps the audience engaged and cognizant of each level as the team experiences various disruptions all at the same time.
For example, on the second level of the dreamworld the team is in a van falling into the river from a bridge. While ordinarily this would take seconds to happen, the van is suspended in slow motion as the events occur in the other levels where time takes longer to play out. Nolan cuts back and forth to keep establishing the time sequence, and it all works rather well and keeps the audience truly involved and thinking throughout.