During the helicopter ride, Saito interrogates Cobb and Arthur about “inception”: the act of planting an idea in a dream, rather than extracting it. Arthur maintains it is impossible, and Cobb seems ready to walk away from the treacherous deal Saito is proposing. When Saito offers Cobb the chance to return to America despite his criminal past and reunite with his children, Cobb asks how complex the idea they need to “incept” would be. Saito explains that he wants the son of his rival to decide to break up his family empire’s company. When Cobb asks for a guarantee from Saito, Saito can only offer a promise. Against Arthur’s advice, Cobb accepts the offer.
On the way to Paris to find a new “architect,” Arthur insists that inception is impossible, but Cobb tells him he has done it before. In Paris, Cobb meets with his mentor and father-in-law, Professor Stephen Miles, who helps Cobb keep in touch with his children but disapproves of Cobb’s life as a thief. Admitting that he can no longer build dreamworlds due to Mal’s intrusions, Cobb asks for an architect, and Miles introduces Cobb to an architecture student named Ariadne.
Cobb first challenges Ariadne to draw mazes, then induces her into a shared dream where the two sit at a Parisian cafe. As Cobb explains the creative logic of dreams, explosions rock the street, waking Ariadne up in a loft. Arthur tells her that “dream sharing” began as military technology, and that one hour in a dream is equivalent to five minutes in reality.
Ariadne re-enters the shared dream with Cobb, who explains that whereas she is designing the surfaces of the world, his subconscious is populating it with “projections.” Ariadne begins to experiment, folding the city back on itself, and walking up walls, which Cobb warns will make the projections increasingly hostile. Cobb warns that building dreamworlds from memory rather than imagination is dangerous, and when Ariadne asks how Cobb knows this, Mal appears and stabs her, waking her up. In the loft, Arthur tells Ariadne she will need a “totem” —a small, unique object that lets its owner know they are not in a dream. Disturbed by Cobb’s subconscious, Ariadne leaves, but Cobb reassures Arthur by saying she’s a natural and will be back. Cobb then tells Arthur he is risking going to Mombasa, which is Cobol territory, to recruit a forger named Eames.
Cobb meets Eames in a casino and quizzes him on the process of inception. Eames explains that to be effective, the incepted idea must be simple, and recommends a chemist named Yusuf. Eames points out that Cobb is being followed, and distracts the Cobol tail so Cobb can escape. In hot pursuit, Cobb is saved when Saito suddenly pulls up in a sedan.
Back in Paris, Ariadne returns to the loft as Cobb predicted, and Arthur introduces her to “paradoxical architecture”—maze-like structures that will help them avoid projections in dreams. Arthur tells Ardiane that Cobb has been haunted by the ghost of Mal, who is dead. Meeting with Yusuf along with Eames and Saito in Mombasa, Cobb asks for a sedative so that his team can create a three-layered dream. Yusuf takes the men into a basement where sedated dreamers have decided to spend their lives asleep. Yusuf gives Cobb a sample of the sedative, and Cobb dreams of Mal.
Gathered on a rooftop, Cobb, Eames, and Saito discuss inception. Saito maintains that the world will be better off if his rival’s energy company fails. Eames inquires about the relationship between CEO Maurice Fischer and his son, Robert Fischer, which Saito describes as “complicated.” Glimpses of the Fischer estate reveal that Maurice is on his deathbed, and that Robert’s uncle Peter Browning is angling to control the company.
The main plot of Inception is essentially a heist film in reverse: the story of a team of criminals who attempt to infiltrate a "compound" of sorts in order to plant something, rather than steal it. What Cobb's team is planting—or, "incepting"—is an idea in the mind of Robert Fischer that convinces him to break up his father Maurice's estate. In order to make the idea seem self-generated, Cobb and his associates must seed the idea over three layers of Robert's dream. Nolan dedicates the second act of the film to Cobb's attempts to assemble a team of dream-sharing technicians—the chemist Yusuf, the forger Eames, and the architect Ariadne—who will join him, Arthur, and Saito.
Nolan uses Cobb's scenes with newcomer Ariadne as a way to explain for the audience the rules of dream-building and dream-sharing in greater detail, as well as probe the moral implications of this highly guarded technology. Ariadne begins the sequence in a state of bewilderment and awe at the hyper-realistic veneer of the dreamworld that she and Cobb are able to share. Nolan once again uses extreme slow-motion during the explosions to lend simultaneously dreamlike and hyper-real visual effects. Like the opening of the film, Cobb and Ariadne's shared dream begins in medias res at a diner at which she cannot remember arriving. By the end of the sequence, however, Ariadne is more troubled by the violent, unresolved content of Cobb's subconscious.
In this section of the film, Cobb and Arthur describe to Ariadne the many ways in which dream-sharing distorts time and space. Dreams, and dreams-within-dreams, not only unfold at progressively slower rates of time in relation to reality, but can also keep past memories alive. Cobb's early warning to Ariadne never to use personal memories to build dream-worlds is ironic given that he is powerless to control Mal's violent advances in his subconscious. Cobb's history with and vulnerability to Mal is a key plot device, one that the film foreshadows when Cobb tells Arthur that he has in fact successfully performed an "inception" once before.
Dreams can also distort space, which Arthur models for Ariadne by showing her how a Penrose staircase is possible in a dream. The character Ariadne is named after the Cretan princess who ruled over Minos's labyrinth and helped Theseus kill the Minotaur. As an architect of labyrinthine dream environments, Ariadne takes on a similar role for Cobb, who begins their lesson by asking her to draw pen-and-paper mazes. Nolan uses the concept of "paradoxical architecture" in creative and audacious ways in Inception's visual design, such as the scene where Ariadne folds the city of Paris back onto itself, or when she and Cobb seamlessly begin to walk up walls.
Nolan uses symbols like mazes, riddles, and labyrinths to symbolize the elusive complexity of the dreaming mind, which can produce a world that for many is indistinguishable from reality, and more seductive. The scene where Yusuf introduces Cobb to the sleepers in his basement, who prefer the shroud of the dream-world over reality, raises existential questions about the true nature of reality that philosophers have debated since Plato's "Allegory of the Cave." The scene also evokes the lotus-eaters of Greek mythology, who tempted Odysseus's men with an easy, narcotized life of sedation. As an allegory of cinema, Yusuf's inveterate sleepers might resemble habitual moviegoers, "dreaming" together through technology in darkened cineplexes.