The movie starts with Dom Cobb (Leonardo) washed up to a beach. He is injured and seems delirious. He opens his eyes once and sees two kids playing on the beach. Some men with guns take him to an old man telling him Cobb is alone with a pistol and a small brass top. The old man tells Cobb that he had once known a man who had that top with him and his name was Cobb.
The scene shifts to Cobb, Arthur (Joseph Gordon-Levitt) and Saito (Ken Watanabe) having a quiet dinner, while Cobb is explaining that if Saito wants to protect himself from Extraction (stealing an idea from dream) he would have to let Cobb into his mind. Tell him all his secrets. Saito seems uncertain and a little suspicious and leaves the room saying he will consider the proposal. Cobb and Arthur move out of the room and Arthur says Saito suspects something is wrong. Cobb says 'All the secrets are in that room. He looked right at it when i said safe'. Arthur then points out a woman in the crowd to Cobb and asks 'what is she doing here?'. Cobb then goes to meet her and he calls her 'Mal'.
Next scene we see Mal and Cobb in a room, Cobb makes Mal sit on a chair and ties a rope to the leg and ties to climb out of the window down the building. We get to know from their conversation both know each other very well. Cobb climbs into the same room where he was previously having dinner and opens a safe and takes an yellow envelope. Just then Saito and Mal enter the room with guns. Cobb asks Saito if Mal told him the truth. Siato says 'what truth? that we are all in a dream?' Then we see Cobb, Arthur and Saito actually sleeping in a room and a fourth guy monitoring them. Outside the house they are sleeping in, mobs are rioting and getting closer.
Back to the dream, Mal brings in Arthur and shoots him in the leg, because killing someone in the dreams will only wake them up, but pain is part of the brain and it's the same in the real or dream world. Cobb shoots Arthur in the head and kills him, and Arthur wakes up in the house. In the dream, all hell breaks lose, fighting and gunshots and buildings crumbling, Cobb tries to read the papers in the envelope.
In the sleep house Arthur tells the monitor (Nash) to wake up Cobb, who pushes Cobb into a bath full of water. With amazing dream visuals of water breaking through walls and entering into dream in sync with the real world bath water, Cobb wakes up, and so does Saito. There is a struggle between all, and Cobb says Saito didn't put all the secrets in the safe and he needs them for the men who hired him, he threatens to kill him. Saito then realizes even this is a dream and it is the 4th guy's (Nash's). As the rioting mob from outside breaks into the house all 4 of them wake up in a train. Cobb and Arthur and the dream architect leave the train. Saito wakes up alone with a teenage kid in his compartment.
Next scene we realize, Cobb has been running from US authorities. He has two kids and they are with their grandmother in the US. Cobb and Arthur decide to lie low for a while thinking their employer would not be very pleased about the botched job. Saito catches up with them with his men and offers a proposal.
He wants Cobb to carry out Inception (putting an idea successfully into someone's mind). If he succeeds, Saito will make sure all charges against him are dropped and then he can return to the US and go back home to his kids. Cobb agrees to this in desperation, knowing that Inception - unlike Extraction - is an extremely complicated process.
To assemble the dream team he goes to meet his father-in-law (Mal's father), named Miles (Michael Caine), who is a professor in a university and asks for a good architect. Miles resists at first because he thinks Cobb is going to use the architect for his espionage, but when Cobb tells Miles that it is the only way for him to get out of the mess he is in and that it should be left on the architect to decide whether he/she wants to take the job or refuse it, Miles decides to help him. Miles introduces him to Ariadne (Ellen Page), saying she is better than him. Cobb then takes her into a dream and makes her realize in the dream that you can design your dream environment - be an architect. She starts building the world as she goes walking in the dream. Cobb then explains all the nuances of designing a dream, that you can allow someone's subconscious into your dream and that is why Cobb is there, and all the people around him are his projections or shades, where as the dream world itself is designed by Ariadne.
As she keeps changing the worlds he warns her that too much of change warns the subconscious that its not real but a dream, and people in the dream start staring at her. She creates a bridge out of memory, which Cobb recollects as one of the places at which he spent time with Mal, he worriedly tells her creating something exactly like a memory brings out the sides of subconscious that they won't like. Suddenly everyone grabs them and Mal walks up to Ariadne and stabs her. Both Cobb and Ariadne wake up in a workhouse. She freaks out completely and leaves. Cobb tells Arthur that she will be back and he goes to get a forger (imitator) and with Saitos help brings Eames (Tom Hardy ) and Yusuf (Dileep Rao), a chemist, into the team. Yusuf tells them about a sedative which is used for complicated layered dream.
The team decides to do a three layered dream set, with the third layer being the crucial point where the idea will be planted. The target is Robert Fischer, Jr., a multibillion dollar oil company heir. Fischer's father is dying and he is preparing to assume control of his father's vast empire. His godfather is Browning, a key part of the empire. Saito wants Cobb to plant an idea in Fischer's head that will split up his father's company. Because he wants to verify that the idea is implanted, Saito will accompany the team into the dreams.
While prepping for the job Ariadne learns that Mal is Cobb's deceased wife and actually sees a bit of his past while in his dream one day. He has issues that are making him unable to architect any dream without bringing in Mal as a spoiler. Ariadne warns him he should tell others. Cobb tells her that, this is why he is avoiding knowing the architecture of the dream she is creating so he doesn't know the secrets so that Mal cannot learn them either.
Arthur explains to Ariadne about totems, something which tells a person whether he is in a dream or real life. Cobb has a small brass top (same as the one we see in the beginning of the movie). Arthur explains that in a dream a spinning top would never stop. Arthur has a loaded dice. Ariadne sets to build a totem, a chess piece, for herself.
When Fischer Sr. dies, the team decides to execute the plan on a plane to LA in which Fischer Jr. is carrying his father's coffin. Saito buys the entire airlines, and the only passengers in the first class section are the dream team and Fischer and the monitor who is the airline stewardess. Cobb slips Fischer the powerful sedative. The dream team quickly hooks themselves up and enters the dream.
10 hours of real world time gives 1 week in the first level dream, 6 months in the second level and 10 years in third level dream. Their wake up mechanism is called the kick. It is the jolt received when suddenly falling or hitting a surface in their dreams.
They enter the first level of dream. It is raining heavily. Arthur, Saito and Eames kidnap Fischer and take him to a warehouse. On the way Cobb's subconscious brings a freight train right in the middles of the road and they get ambushed on the way by gunmen, and Cobb explains that Fischer might have been trained against dream extraction and these are his defenses. They some how reach the warehouse but Saito gets hit in the chest with a bullet. When Eames tries to kill him to put him out of his misery, Cobb stops him saying the sedative they all took will not allow them to wake up. If you die before the sedative wears off you just go into a limbo, a lower world of dreams which could mean eternity to a mind stuck in there. By the time you wake up your mind could be scrambled and you lose all sense of reality.
When Ariadne confronts him about being the cause of the train in the middle of the road, he confesses that he can't seem to get Mal out of his dream. He tells her the story how she died. During their last dream together, something goes wrong and they get stuck in limbo. Cobb tries to convince Mal that this isn't real and they should go back. They have spent 50 years in the limbo world that they constructed. Mal finally agreed to come back. But when they did, Mal kept thinking they are still in dream and that dying is the only way to wake up and return home. She calls Cobb to a hotel room on their anniversary and tells him she sent a letter to their attorney saying Cobb is trying to kill her. She says she did that so it would be easy for him to die with her. She jumps off the balcony and dies. With authorities chasing him, ready to imprison him for the murder of his wife, Cobb decided to leave his children and flee the country.
Eames impersonates Browning and screams like he is being tortured. Arthur and Cobb talk to Fischer wearing masks. They tell him there is a safe in his office and they want the code to that. Fischer denies knowing of any safe. But when he hears Browning (really Eames) scream he asks them to let him speak to him. Eames impersonating Browning tells him his father wrote a second will dissolving the empire and he, Fischer can use it if he wants to. Fischer wonders why he would want to dissolve his empire, Browning says maybe his father wanted him to build his own, and that he loved him. Fischer says his father didn't love him because his last words to him were that he was 'disappointed'.
Fischer's defenses start closing in on the warehouse. They decide to do the second level dream in a van. Yusuf is to drive the van and he should drive it off the bridge. That free fall is their trigger - the kick- to come back from dream. They enter the second dream sequence constructed by Ariadne and Arthur. It is a restaurant in a hotel where Cobb meets Fischer as his security officer. He makes him realize its a dream because he doesn't remember how he ended up there and the gravity keeps shifting. (Meanwhile, in dream stage 1 Fischer's defenses are chasing and trying to destroy the van.) Fischer's own mind projects Browning in the dream as the bad guy who is behind the kidnap, the projection Browning confesses saying Fischer empire is all he ever worked for and he couldn't let him destroy it with the will in the safe.
Cobb convinces Fischer that he should enter a dream and discover Browning's secrets. They all gather in a hotel room and enter the third level.
Third level of dream is set in snow mountains, where the safe is in a fortress guarded heavily by armed men. Cobb tells Fischer that he must go to the house and open the safe. Eames and the rest draw the guards away.
Dream state 1: van drives off the bridge. 10 seconds left.
Dream state 2: No gravity situation in hotel. Floating Arthur - the monitor for Dream state 3 - is getting everyone together after devising a way to improvise a "kick". 3 mins left.
Dream state 3: Fischer is struggling to get to the safe. 60 mins left.
After a long struggle and shootout in the snowy fields surrounding the fortress-like house, Fischer finally reaches the safe. But just then Mal appears. When Cobb hesitates to shoot her , Mal shoots Fischer, killing him. Cobb then shoots Mal. Everybody gathers around Fischer. Saito dies. Ariadne proposes they must go deeper - to limbo - and get Fischer out. He must be in Cobb's limbo as Mal is nothing but a projection of his mind.
Ariadne and Cobb enter the 4th level dream - limbo. This is the world in which Mal and Cobb built and inhabited for 50 subjective years. They go to the house and find Mal sitting in the kitchen. Mal accuses him of leaving her, telling him he promised they would grow old together. She tells him he is confused and the place they are in is the real world. He should stay there. Cobb tells Mal that she isn't real and Cobb has been feeling guilty a long while. The brass top is actually Mal's.
Jumping to flash back when they were stuck in limbo. Mal locked the top away in a safe. Cobb found it one day and put the idea in her mind that that was not a real world. He left the top spinning in the safe. When the idea became a strong belief, Mal finally agrees to leave the world, they both lie on the train track to die and wake up in their house. But the idea - that her world is not real - takes possession of her mind and she dies in real life, thinking she would finally wake up and go home. And that's how Cobb knows the whole concept of Inception is possible because he planted the idea that her world was not real in her mind.
In the kitchen in limbo - dream state 4 - Cobb tells Mal, this is not real and that he is going back.
Dream state 4: Cobb tells Ariadne that he will have to find Saito because he has died ( in upper level dreams) and is now down there with him someplace in limbo. Ariadne finds Fischer and pushes him off the cliff and falls down herself, providing the "kick" to leave that dream.
Dream state 3: Eames uses a defibrillator to revive Fischer who wakes up and goes into the safe. He finds his dying father there, who tells him he is disappointed that his son tried to be like him. He wants him to be his own person. Fischer decides to break up the empire. He takes the will from the safe beside the bed. Ariadne wakes up.
Dream state 2: Arthur has pulled everyone into lift and creates a free fall situation with explosives, providing the next "kick.".
Dream state 1: The van hits the water, providing the final kick. Every one wakes up except Cobb and they swim to the surface. On the shore, Fischer talks to Browning (Eames) about his new plan for the company.
Back in limbo, there is a repeat of the opening scene of the movie. Cobb washes up on the shore and is barely coherent. He is taken to the old man and we realize it is Saito. Cobb tells him he should come back. Saito takes the gun in his hand.
Next scene, Cobb is waking up in the plane and so is Saito. Saito makes the call to get the authorities off Cobb's back. Everyone deplanes. Cobb goes through security and customs without a hitch. He sees the other team members collecting their luggage. Miles is there and welcomes him and takes him to his old house where he again sees his children playing in the backyard.
Cobb spins the top on the table to see if this is real or another dream. Just then the children turn and see him, shout "Daddy!", and start toward him. He goes outside to hug them and the camera focuses on the top still spinning. The top is still spinning but wobbles slightly, as if it were about to topple. At that point the screen blacks out, leaving open the question of whether Cobb's long-sought homecoming is reality or dream.
