A young man, exhausted and delirious, washes up on a beach, looking up momentarily to see two young children (Claire Geare and Magnus Nolan) playing in the sand before he passes out. An armed guard (Tohoru Masamune) discovers him and has him brought to a large, seaside palace where the proprietor, an elderly Japanese man, is told of the stranger's arrival. The only objects found on him were a handgun and a brass top. The old man allows the stranger entry. He is dragged in and given some food which he struggles to eat as the old man picks up the brass top and says, "You remind me of someone...a man I met in a half remembered dream. He was possessed of some radical notions." The stranger looks up in realization as the scene shifts...
Dom Cobb (Leonardo DiCaprio) and his partner, Arthur (Joseph Gordon-Levitt), speak to a prospective client, Saito (Ken Watanabe), in an eerily similar dining room about the services they provide; specifically protection against thieves specialized in extracting valuable information from a subject while they're dreaming. Cobb explains that, when one is asleep, one's mind is vulnerable to attack and what he can do is train Saito's mind to subconsciously defend against extractors to protect whatever secrets he may be hiding. He backs up his claim by revealing that he is the most skilled extractor there is and knows all the tricks involved. Appearing skeptical, Saito stands to leave, telling Cobb he will consider his proposition, and exits the room to join a small party in the main hall.
Arthur casts Cobb a glance, saying, He knows as the room begins to shake. They walk to an outside balcony where other partygoers mingle and Arthur points to a woman nearby, asking Cobb what she's doing here. Cobb assures him that he'll take care of it and to proceed with the job. He knows where Saito's secrets are; he glanced over at a safe the minute Cobb mentioned the word.
Cobb approaches the woman who asks if he misses her. He responds that he does but can't trust her anymore. They retreat to a private room where Cobb ties a rope to the leg of a chair and tosses the end out of the window. He tells the woman, Mal (Marion Cotillard), to take a seat as she asks him if the children miss her. Cobb pauses a moment before saying, "I can't imagine." He then repels out the window to a ledge below, nearly falling when Mal leaves her seat. He breaks into the room below and accesses the safe, swapping out the manila folder inside for another, as the lights in the room turn on. He turns with his gun out to see Mal aiming a gun at him and standing beside Saito and a guard holding Arthur. Cobb asks Saito if 'she told him' as he slides his gun across the table. Saito responds, "That you are here to steal from me, or that we are actually asleep?"
This proves true: the three of them are hooked up to a PASIV (Portable Automated Somnacin IntraVenous) device which feeds them the sleep drug Somnacin, keeping them all asleep and allowing dream-sharing. They are watched over by Nash (Lukas Haas), another of Cobb's partners. He checks Saito, sleeping on a bed, before going into another room to check on Arthur and Cobb, both asleep in chairs. Cobb's chair sits above a tub full of water. Explosions and shouts outside get closer as a mob of rioters moves down the street.
Inside the dream, Mal holds her gun to Arthur's head but Cobb tells her the threat is empty, as he hands the manila folder over to Saito, since shooting him will only wake him up. Mal smiles in agreement but says that pain is only in the mind and perceived as real as she shoots Arthur in the knee, causing him to cry out. Cobb dives across the table and retrieves his gun before shooting Arthur in the head. He then dashes out of the room under gunfire from Saito's guards.
Arthur wakes up and instructs Nash that things are falling apart but Cobb still has time to finish the job before he checks on the still sleeping Saito.
Saito frantically opens the manila folder as the dream begins to collapse and shouts in anger as he finds blank pages inside. Cobb manages to hide away for a moment to look at the contents of the real folder which he'd hidden in his jacket, gazing over the confidential files as the building crumbles around him. Saito is crushed by debris and wakes up in the apartment, unseen to Arthur as he reaches under his pillow. Arthur tells Nash to wake Cobb by giving him the kick. Nash pushes Cobb backwards into the tub and, as he hits the water, Cobb's dream is flooded with massive waves cascading through the windows before he wakes up. Saito, having reached under his pillow for his gun, grabs Arthur but is subdued by Cobb who tells him that not all the information he needed was in the file he stole. Saito laughs and claims that all the information he had was in the file because he knew of Cobb's ruse all along. He allowed Cobb and Arthur into his mind as part of an audition which they failed, saying that 'your deception was obvious.'
Asserting that his employer, Cobol Engineering, won't accept failure, Cobb throws Saito on the floor and demands that he tell them what they need to know about his expansion project. With his face pressed into the carpet, Saito begins to laugh again and reveals that he is familiar with the material of the carpet; it is supposed to be made of wool instead of polyester. Thus, he comes to the conclusion that he is still sleeping.
Sure enough, Saito, Arthur, Nash, and Cobb are all asleep in the car of a train, watched over by a young man named Tadashi (Tai-Li Lee) who monitors the time remaining on the PASIV device. He places headphones over Nash's ears and plays music as a cue that their time is running out. The music plays faintly within the dream but enough that Nash can hear it. As the rioting mob outside draws nearer, Saito commends Cobb on creating a dream within a dream but becomes confused at his inability to control this dream. Nash reveals that they're not in Saito's dream...they're in his. The mob breaks through the door, attacking everyone in the room, and Arthur, Nash, and Cobb wake up on the train. Cobb berates Nash, the architect of the dream, for designing the carpet wrong and throws Tadashi a wad of money before leaving, telling them 'every man for himself'. Saito wakes up moments later to find himself alone in the car, save for Tadashi who's resumed a casual pose, but smiles wryly to himself. In his apartment, Cobb spins his brass top and takes his gun, pointing it at his temple as the top spins. When it falls, he breathes a sigh of relief and puts the gun down. His phone rings and he picks it up to hear his two children, James and Phillipa (Johnathan Geare and Taylor Geare), on the other line with their grandmother. They ask when he's coming home and he responds that he can't because of work. When James asks if their mother is with him - an image of Mal crosses his mind - Cobb pauses and tells him that 'mommy's not here anymore'. He tells them to behave and that he'll send presents with grandpa before their grandmother hangs up.
Arthur knocks on the door and tells Cobb their ride is on the roof. Cobb decides to fly to Buenos Aires to lie low in lieu of their failed job for Cobol while Arthur says he's returning 'stateside'. Cobb asks him to send his regards as they open the door to the helicopter and see Nash, beaten and bruised, and Saito waiting for them. Apparently, Nash had tried to sell out Cobb and Arthur for his own safety but Saito has other interests. He offers Cobb a job performing 'inception' for him, something Arthur claims is impossible. Cobb, however, says that it's not impossible but extremely difficult since it involves planting an idea in someone's mind rather than extracting one. He turns away, insisting that he'll find a way to resolve relations with Cobol himself but Saito then asks him if he wants to go home to his children in America. He promises Cobb that, if he succeeds, all he will need to do is make a phone call and the charges keeping Cobb out of the country will be dropped. Desperate, and to Arthur's exasperation, Cobb accepts.
They board the helicopter with Saito while two thugs carry Nash away to an uncertain fate. En route, Arthur explains to Saito the nature of inception, telling him that simply planting an idea in someone's head does not guarantee that the idea will take. The subject may very well discover that the idea is not theirs and reject it. True inspiration, Arthur claims, is impossible, despite Cobb's thoughts otherwise. Saito shares with them his reasoning for the job; he needs the CEO of a competing energy conglomerate to split up his father's company, ensuring Saito's own Proclus Global complete domination over the energy production industry. Despite the daunting task, Cobb agrees to perform the job. Saito drops them off at the airport and advises Cobb to choose his team wisely.
Cobb travels to Paris where he meets up with his father-in-law, Miles (Michael Caine), a professor at a university and the one who taught Cobb and Mal about dream-sharing and designing dreams. Cobb asks for an architect, one as good as he, and Miles points him to someone better. He introduces Cobb to a graduate student of his, Ariadne (Ellen Page) who is immediately put to the test by Cobb to design a maze that takes one minute to create and two to solve. After impressing Cobb with her skills, he tells her more about his line of work and what is required of her. Her job as architect will be to design dreams and create virtual mazes for the dreamers subconscious to inhabit while allowing Cobb and his team to work. As they talk outside a café, Cobb attempts to make Ariadne aware that they are actually in a dream. The realization causes Ariadne to panic and the dream violently collapses. When they awake, Ariadne finds they are in Cobb's warehouse workshop where Arthur is monitoring them. Ariadne shows surprise when Arthur says they'd been under only five minutes when it felt like hours. Cobb explains that the mind functions faster in a dream, so time moves slower. They go under again and Ariadne is given the opportunity to creatively alter the physics of the dream. Her architectural wonders cause the people in the dream - projections of Cobb's subconscious - to search for the intruder - Ariadne - like white blood cells drawn to a virus. When Ariadne makes the mistake of creating a bridge from her memories, Cobb recognizes it and his subconscious reacts as a mob, separating Ariadne from Cobb until Mal appears and stabs her.
She wakes up and Cobb rushes to the restroom while Arthur explains that Cobb's subconscious became aware of her as an invasive being and she was unable to wake up right away because there was still time on the clock. The only way for her to wake up was if she died. Cobb takes out his top and spins it, sighing as it topples over. Ariadne leaves the warehouse, angry and refusing to open her mind to Cobb if his subconscious is as tormented as it seems. Cobb returns to the room and assures Arthur that she'll be back but he needs to make a trip to Mombasa to recruit an old friend to the team. He finds Eames (Tom Hardy) gambling at a bar and offers him a place on his team as a forger/imitator. Eames agrees before telling Cobb he's being tailed, pointing to two men at the bar. Cobb recognizes them as Cobol thugs who must be aiming to collect the bounty on his head for the botched Saito job. Eames creates a distraction while Cobb escapes, leading the thugs on a chaotic chase through the city streets. At the last moment, Saito pulls up in a limousine and picks up first Cobb, then Eames. He explains that he's been tracing Cobb to protect his investment.
Eames takes them to a local chemist he knows who experiments with Somnacin and who might be an asset to their team. Yusuf (Dileep Rao) listens as Cobb explains that his job may require the use of a three-layered dream. Yusuf says that this would be otherwise impossible, due to the instability of dreams the further down you go, if not for a special solution he's concocted with a powerful sedative. To show its effectiveness, Yusuf takes them downstairs where they see dozens of men sleeping under the watch of an old man (Earl Cameron). He tells Cobb that these men come here to 'wake up'; dreaming has become their reality. With the aid of the sedative, their sleep is deep and stable and they are able to dream for what feels to them like years. Cobb tries the sedative himself and is impressed with its affects, though shaken after waking from a vivid dream with Mal. He convinces Yusuf to join his team.
Meanwhile, Ariadne returns to the workshop where she tells Arthur that she meant to stay away but couldn't resist the pure creation involved in architectural dreaming. Arthur takes her into a dream and introduces the notion of creating paradoxes, such as the Penrose steps. He also reveals that Mal was Cobb's wife and has since passed away. Despite Mal's malevolent nature within the dreams - her existence now only as a projection of Cobb's - Arthur tells Ariadne she was lovely in real life.
The team bands together and decides that they will create a three-level dream with the third level containing the planted idea. The target in mind is Robert Fischer (Cillian Murphy), the CEO and heir of Fischer Morrow, whose father, Maurice (Pete Postlethwaite), is slowly decaying with illness. Eames targets Robert's godfather and business partner, Peter Browning (Tom Berenger), to get a better grasp on the father-son dynamic and to use his imitation skills at their best. The team decides that, in order to get Robert to split up his father's company and because of their complicated relationship, a positive idea will trump a negative one; 'my father wants me to be my own man.' Saito oversees most of the plans and, because he wants full verification of any success or failure, decides that he will accompany the team into the dreams as a 'tourist'.
As the team prepares over the next few weeks, Arthur shows Ariadne the significance of 'totems'; small, personal objects that enable a person to differentiate between dreams and reality. Cobb's totem is his top, which topples over in the real world and keeps spinning in a dream. Arthur's is a loaded die and he instructs Ariadne to create one for herself that only she can touch to ensure its validity. She creates a semi-hollowed bishop chess piece as her totem. Wanting as much time as possible to complete the job, the team decides, with Saito's help, that they will perform the job on a Boeing 747 during an international flight from Europe to Los Angeles, a 10 hour flight. This will give them a week in the first stage of the dreams. Saito reveals that he's bought the entire airline, making the job neater and without having to buy out certain sections of the plane for access.
One evening, after a day of formulating plans, Ariadne finds Cobb dreaming alone in the workshop. Curious, she hooks herself up to his machine and finds herself descending in an elevator. She sees Cobb sitting in a living room with Mal who quickly detects Ariadne's presence. Cobb gets up and joins Ariadne in the elevator, leading her through some of the levels that Ariadne discovers are each specific memories; something he originally told her never to do. Horrified after seeing Cobb's torment over leaving home without saying farewell to his children, Ariadne takes the elevator alone to the last level where she sees a hotel room, the contents of which are strewn on the floor. She steps on some broken glass and Mal looks up from the couch, asking what she's doing here. Ariadne tries to explain that she just wants to understand but Mal becomes defensive and picks up a shard of glass. Cobb arrives and takes Ariadne back to the elevator as Mal rushes forward, shouting at Cobb that he didn't keep his promise.
Ariadne and Cobb exit the dream and Cobb explains that the reason he can't go home is because of Mal's death, because it was thought that he killed her. He thanks Ariadne when she doesn't ask him if he did or not but she does warn him that he's mistaken if he thinks he can cage Mal like that. He needs to release his guilt over her death. Ariadne convinces Cobb to allow her to go with the team into the dreams because he needs someone who understands what he's going through. At that moment, Saito and Arthur arrive and announce that Maurice Fischer has died and Robert will be accompanying the body to the States in a few days.
The team boards the flight and sits with Robert in a sectioned-off first class cabin. Cobb returns Robert's passport, pick-pocketed by Arthur, as a conversation starter and then drugs his water before proposing a toast in his father's honor. Within moments, Robert is asleep and the paid-off flight attendant (Miranda Nolan) assists setting up and activating the PASIV device. All together, the team descends into, first, Yusuf's dream. It is raining heavily in New York City as members of the team are picked up. Arthur and Saito commandeer a taxi, pick up Robert and Eames and then Yusuf as Saito holds a gun to Robert as part of the kidnap ruse. Cobb and Ariadne follow in a separate car until a train suddenly barges down the middle of the street, hitting their car and temporarily stalling them. Gunfire then opens on the taxi and the team is forced to take immediate evasive maneuvers, hiding out in a warehouse where its discovered that Saito has been shot in the chest.
Robert is taken into another room while Saito is laid on a table. Before Eames can shoot him to end his misery and wake him up, Cobb stops him and explains that, due to the sedatives they've taken, they won't wake up if they die. Instead, they'll be sent to limbo; a shared dream-state of raw subconscious where time is practically non-existent. The team is angered by this, wondering why they've taken such a risk, as they contemplate what to do about the armed forces closing in on their location. They find out that Robert's subconscious has been trained to fight against extraction and the projections attacking them are part of that defense. With Saito's condition deteriorating and unable to wait much longer due to the approaching defense projections, the team decides they need to complete the job as quickly as possible.
Ariadne confronts Cobb about the control he has over his own subconscious and he confesses that he can't keep Mal out of his head. He tells her that they had been experimenting with dream states and wanted to see how far down they could go into their subconscious. They wound up in limbo together, unable to leave because of the time remaining on their PASIV clock. They recreated their lives, spending years worth of building. After so long, they began to perceive limbo as their reality. After something like 50 years, Cobb and Mal killed themselves on train tracks to bring them back to reality. Despite returning to the real world, Mal continued to believe that she was still dreaming and believed that dying was the only way to 'wake up', but she refused to leave without Cobb; she loved him too much. On their wedding anniversary, Cobb went to the hotel room they always stayed in to find it trashed and the window open. Outside, on an opposite ledge, sat Mal who revealed to Cobb that she filed a letter to their attorney expressing a fear for her life, effectively framing him in the event of her death and forcing him no other way than to join her. Cobb refuses to jump and attempts to bring Mal to her senses, but she ignores him and jumps to her death. Since she declared herself legally sane by three psychiatrists, Cobb's case for his innocence is overruled by the outstanding evidence against him. With no other choice, Cobb leaves his children behind with their grandmother and flees the country.
Ariadne tries to convince Cobb that Mal's death was not his fault and that he needs to focus on the mission. Eames prepares himself as an impersonation of Browning while Cobb and Arthur interrogate Robert, demanding to know the combination to his father's safe. They pressure Robert by using 'Browning' as leverage. Eames shouts from another room as if being beaten before he is brought into the room with Robert to try and get him to remember. He tells Robert that the safe contains an alternate version of his father's will, one that will dissolve the company if Robert chooses so. 'Browning' tells Robert that his father loved him and wanted him to build something of his own. As Robert's defenses close in on the warehouse, Robert reveals that one word he could decipher out of his father's last words was 'disappointed', convincing him that his father didn't love him.
With the warehouse in danger of being infiltrated, the team pressure Robert once again for a combination. He tells them a series of random numbers that come to mind before they load him into a van and drug him to sleep. They all get in and prepare to enter the second level of the dream while Yusuf drives the van away from the pursuing projections.
In Arthur's dream, Cobb resolves to use 'Mr. Charles', a method in which he introduces the subject to the fact that he's dreaming in order to garner trust. He meets Robert at a bar and tells him that he is there to protect him and someone is trying to access his mind. He convinces Robert that he's dreaming by introducing the strangeness of their surroundings and calms Robert to control them. He helps Robert remember that he's been kidnapped and leads him to a hotel room (the first few numbers of which match the first digits Robert thought of for the combination) where the rest of the team regroups. Saito is in better health in this level of the dream, but soon begins coughing. Robert's projection of Browning arrives and he confesses that he was the one behind the kidnapping; he couldn't allow Robert to access the safe and destroy everything that his father built. Cobb suggests to Robert that they enter Browning's dreams to figure out what was really in the safe so that Robert can decide for himself. Robert agrees, now unknowingly assisting in his own inception and the team is hooked up again, this time with Arthur remaining behind to watch over them and administer a synchronized kick when its time.
As the team goes into the third dream, in actuality Robert's, Arthur is forced to fight off more of Robert's defensive projections while in the first dream Yusuf continues to drive the van. The third dream is set in snow covered mountains where Robert's safe is heavily guarded in a mountainside fort. The team splits up to draw the guards away. Cobb goes with Ariadne, Eames travels alone to ward off the guards, and Robert and Saito begin ascending a mountainside to access a blind side of the fort.
Meanwhile, Yusuf momentarily loses control of the van and it tumbles down an incline, the tumbling effect translates into Arthur's dream as he fights off Robert's projections, the environment around him spinning as gravity reverses and then rights itself. As Yusuf continues driving, he becomes cornered on an elevated bridge with one car full of projections stuck with him. Yusuf plays music through Arthur's headphones to warn him of the incoming kick before driving backwards off the bridge. Upon impact, the force sends Arthur in his dream flying and, as the van plummets in mid air, there is a loss of gravity in Arthur's dream. The impact also translates into the third level of the dream. Saito and Robert look up the mountainside and are forced to cut their lines as an avalanche sweeps down upon them. Cobb realizes they've missed the first kick, but they still have time for the second one when the van hits the water off the bridge.
Van drives off the bridge: 10 seconds left to impact.
Dream 2: 3 minutes to synchronize the kick. Arthur struggles with a way to do this without gravity.
Dream 3: 60 minutes left.
With little time left, Cobb demands to know if there is another way into the fort and Ariadne relents into telling him of a secret underground entrance that Robert and Saito can access. Cobb send them there, all while Saito's condition deteriorates. They finally enter the main room where the safe is located while Cobb and Ariadne watch from a snipers angle. Succumbing to his injuries, Saito is left to lay down, coughing blood as Robert continues forward. However, as Robert comes into Cobb's view, so does another person. Mal drops in from the ceiling and shoots Robert down before Cobb comes to his senses and shoots her. Eames is ordered to the room as Cobb and Ariadne rush to the site. Finding Robert dead, Cobb labels the mission a failure, since the only other place where Robert has gone is limbo. Ariadne, however, convinces him that, if they go into limbo, they'll have enough time to find Robert and bring him back. Eames agrees to use a defibrillator to jump start Roberts heart to help while Cobb and Ariadne go under.
Meanwhile, in dream level 2, Arthur devises a unique plan. He uses phone wires to tie the team together and brings them into the elevator. He lines the outside of the car with explosives, timing down till the kick with the intention of using explosive force to create gravity and instigating a kick.
In limbo, Cobb and Ariadne tour the deteriorating world that he and Mal once built. They see old homes and buildings before they find the one where Cobb knows Mal must be. If they find Mal, they'll find Robert because Mal will want to use something Cobb wants to bring him to her. Sure enough, they enter the apartment and Mal is waiting for them. She tries, again, to convince Cobb that his place is with her in their real home with their children but Cobb reveals a terrible truth, the reason why Mal believed that her dreams were real.
While they were in limbo, Mal had stored away a truth that she didn't want to believe anymore; her totem, placed within her safe, lying on its side, immobile; telling her that her dream, her limbo, was reality. In an effort to save her mind, Cobb broke into her safe and spun the top to convince her that this world was not real. However, he did not know that, once they really woke up, she would continue to believe that. This was how he knew inception would work; because he performed it on Mal first and his guilt over her subsequent death has been plaguing him ever since. He tells Mal that he will stay with her in limbo if she tells him where Robert is and she reveals he's on the porch. Ariadne finds him there and pushes him off as an improvised kick.
Robert comes back to life in dream level 3 with Eames' aid and opens his own safe, finding within an image of his bed-stricken father muttering his last word. Robert acknowledges that his father was disappointed that he couldn't be him, but Maurice says, "No...no. I was disappointed that you tried." Maurice then points to a cabinet where Robert finds the will...and a paper fan his father made for him once as a child. Tearfully, Robert looks up to see his father has passed and breaks down as the van hits the water.
Dream 2, Arthur hits the detonator and the explosives force the elevator down, creating artificial gravity on the team.
Dream 3, a series of explosions set by Eames rock the fort, collapsing the main floor.
In limbo, the synchronization of kicks pulls on Ariadne and she calls for Cobb to join her. Cobb says that he will stay in limbo, but not with Mal. By this time, Saito has died and joined limbo as well. Cobb must find him but promises to return. Ariadne leaps off the side of the building and rides the kicks back to dream 1. In the van, Robert wakes up and escapes the submerged van with 'Browning'. Arthur and Ariadne share an oxygen tank with Yusuf before they escape the van, leaving Cobb.
Robert and 'Browning' make it to shore where Robert reveals that his father really did want him to be his own man and that he's going to do just that and liquidate his father's company. Knowing the mission is a success, Eames drops the Browning mask.
In limbo, Cobb washes ashore where the armed guard finds him. He is brought to the seaside palace where the elderly Japanese man recognizes his brass top. Cobb recalls what he was there to do and calls to Saito, asking him to come back with him and honor their arrangement. The elderly Saito reaches for Cobb's gun.
Cobb wakes up on the airplane and looks around, startled, to see Arthur and Ariadne smiling at him. He looks at the now awake Saito who remembers, picking up his phone and dials. The plane lands in Los Angeles and Cobb nervously moves through customs where security checks his passport, but allows him passage through, welcoming him home. Cobb walks past the rest of the team and Robert, who pauses a moment as if recalling a half-remembered dream. Ahead of him, Cobb sees Miles calling him over. They drive home together where Cobb hesitates before taking out his brass top. He spins it on the table in the kitchen as his children appear at the back door. He runs to them, elated to see their faces again as the top continues to spin, wobbles a bit...and the screen turns to black.