A man named Dom Cobb wakes up on a shore and is dragged into a house belonging to a wealthy Japanese businessman named Mr. Saito. Cobb's partner Arthur appears, and together they explain to Saito that new dream-sharing technology has rendered thoughts vulnerable to theft, advertising their security services to Saito as dream-sharing experts. After Saito leaves, Cobb steals documents from a safe upstairs, and is apprehended by Saito and a woman named Mal. Cobb shoots and kills Arthur, who wakes up in an apartment, where Cobb and Saito are revealed still to be asleep and dreaming. As the dream in Saito's mansion collapses, all of the men wake up in the apartment. Saito reveals that he set up the operation as an audition, and the men wake up again in a moving train.
Later, in Kyoto, Saito then asks Cobb whether he can perform "inception." Saito tells Cobb that if he can plant an idea in the mind of his business rival, Robert Fischer, he can reunite Cobb with his children in the United States. Against Arthur's judgment, Cobb consents to the plan and travels to Paris to find a dream "architect." Cobb's father-in-law Stephen Miles introduces him to Ariadne, a graduate student skilled in creating labyrinths.
Cobb introduces Ariadne to the basic rules and terms of dream-sharing technology, such as a "totem" (an object that lets a person know they are not dreaming) and a "kick" (a physical jolt that brings a dreamer out of a dream). Although Cobb's first rule is never to use memories to build dreams, the memory of his dead ex-wife Mal violently invades his shared dream with Ariadne. Cobb then travels to Mombasa to recruit Eames, a British forger, and Yusuf, a Kenyan chemist, to aid with the mission. Back in Paris, the team convenes with Saito to discuss the plan.
Eames maintains that the incepted idea must be simple, seemingly self-generated, and planted over three layers of Robert's dream. Ariadne becomes worried when she infiltrates Cobb's dreams one night to learn that he still harbors feelings of guilt about his dead wife, Mal, whose memory keeps erupting into his dreams. Ariadne warns Cobb that he cannot keep Mal imprisoned in his subconscious forever. When Robert Fischer's father dies, the team decides to execute their plan on a trans-Pacific flight from Sydney to Los Angeles, on which Robert Fischer is scheduled to be a passenger.
On the plane, Cobb discreetly spikes Robert's drink with a sedative, and the team enters into a shared dream with Robert. They manage to take Robert hostage but are immediately ambushed by a team of rifle-wielding assailants. Saito is gravely injured, and Cobb realizes too late that experts have helped Robert militarize his subconscious against unwanted extraction. To make matters worse, Yusuf's powerful sedative may leave whomever is killed in the dream in "limbo"—unconstructed dream space. Although shaken, the team decides to continue with the plan. Cobb extracts combination lock numbers from Robert, who discusses his cold relationship with his father with his Uncle Peter.
In the back of Yusuf's van, the team descends into the dream's second layer—a hotel lobby, where Eames poses as a female thief to distract Robert. Cobb, himself posing as a security expert, pretends to help Robert by alerting him to the robbery and leading him to a hotel room upstairs. Cobb manipulates Robert into becoming suspicious of the motives of his Uncle Peter, and the three of them sink into a third layer of the dream, which takes place in and around a snow-blanketed fortress.
The team scurries to complete the inception mission when they hear Yusuf, whose van has crashed off a bridge, give the "kick"—Edith Piaf's "Non, Je Ne Regrette Rien." Realizing they have limited time, Ariadne tells the rest of the team the quickest way to infiltrate the fortress. Arthur scrambles in the dream's second layer to improvise an effective "kick" in the hotel where Cobb and the others are asleep, which has become a zero-gravity environment given the falling van in the dream's first layer. In the third layer, Robert reaches the fortress's "strong room" but is shot dead by Mal, who suddenly appears. Cobb thinks the operation is over, but Ariadne convinces him to go one layer deeper in order to save Robert and redeem the mission. Saito dies as Eames places explosive charges around the fortress to act as the third layer's "kick."
Cobb and Ariadne descend into a fourth layer of the dream: a crumbling cityscape that Cobb and Mal built together in the fifty years they spent in "limbo." Cobb confesses to Ariadne that he "incepted" the idea in Mal that limbo was not real, so that she would agree to wake up and be with their children; however, Mal continued to believe that reality was a dream, and eventually committed suicide, leaving Cobb ridden with guilt. In a nearby building, Ariadne and Cobb find Mal and Robert, whom Ariadne wakes up so that he can infiltrate the fortress's "strong room." Cobb tells Mal that she is not real, and Robert inputs a code in the strong room that reveals a large operating room with his dying father inside. Robert's father tells him to live his own life, rendering the inception complete.
As the dreamworld collapses, Cobb tells Ariadne to ride the "kicks" up through the layers of the dream while he rescues Saito. Cobb washes up on a shore and is dragged into Saito's house, just like in the film's opening scene. Saito, now old, recognizes Cobb as a man from a half-remembered dream, and the two of them suddenly wake up on the passenger airliner next to Ariadne, Robert, Eames, and Saito, who have all successfully made it back to reality. Cobb arrives in the United States and is greeted by Stephen Miles, who takes him to his children. Cobb arrives home and goes outside to play with his children, while his totem continues to spin.