With Cobb providing backup, Robert gains access to the fortress’s “strong room” but is followed by Mal. When Cobb hesitates, Mal shoots and kills Robert, before Cobb kills her in turn. Cobb tells Ariadne and Eames that with Robert now in “limbo,” the mission is a failure. However, Ariadne convinces Cobb to descend into “limbo” to find him, where they will have more time, before riding the series of “kicks” back up through the multiple dream-layers.
Eames puts Cobb and Ariadne to sleep, who then wake up on the shore of a vast, crumbling cityscape built by Cobb and Mal. Eames leaves a wounded Saito in charge of protecting a sleeping Robert, Cobb, and Ariadne, so that he can go place explosive charges that will provide the necessary “kick.” As Cobb and Ariadne wander the city, built from pieces of Cobb and Mal’s memories, Arthur tests a controlled explosion in an elevator shaft that will act as the necessary “kick” in his layer of the dream.
Cobb and Ariadne find Mal waiting in the interior of a building-like house. Mal suggests to Cobb that he has been living in a dream, and that his reality is an illusion. Cobb confesses to Ariadne that, in order to convince Mal to leave their fifty-year dream, he incepted the idea in her mind that their world was not real, so they would kill themselves and wake up. However, upon waking, the idea had spread like a virus in Mal’s mind, leading her to commit suicide in the real world, too.
In the fortress, Eames returns to the strong room after placing the charges to find that Saito has died. Mal tells Cobb and Ariadne that Robert is on the porch after Cobb offers to stay with her, but Cobb then enrages Mal by telling her she is just a “shade” of his former wife. As Mal becomes violent, Ariadne shoves Robert off the building, waking him up next to Eames in the fortress.
Robert approaches a fortified wall in the strong room and inputs a code that causes the wall to retract, revealing a large chamber with an operating bed containing his ailing father, Maurice. Maurice tells Robert that he was “disappointed” in Robert only for trying to be like him. Robert opens the safe next to Maurice and finds not only a will, but a small handmade pinwheel he made for his father, and begins to cry.
Arthur and Eames trigger their “kicks” as Yusuf’s van hits the water, and all at once, the multiple dreamworlds begin to collapse. Cradling Mal in the crumbling dreamscape, Cobb tells Ariadne that he will stay behind to try to rescue Saito. Ariadne falls from the building and rides the multiple “kicks” up through the layers of the dream along with Robert and the rest of the team. Robert, Peter, Ariadne, and Arthur swim up out of Yusuf’s van, and Robert explains to Peter that his father wanted him to live his own life. Ariadne tells Arthur that Cobb stayed behind to rescue Saito, not to be with Mal.
Cobb wakes up on a shore, and is once again escorted by armed guards into Saito’s ornate dining room. Cobb urges Saito, who is now an old man, to question reality and to return to a time when they were both young men. As Saito reaches for his gun, Cobb suddenly wakes up on the passenger airliner flying from Sydney to Los Angeles. He looks across at Arthur, Ariadne, and Saito, who have also managed to successfully reawaken. In Los Angeles, Cobb is welcomed back into the United States, as Saito promised, and greeted by Stephen Miles. Stephen takes Cobb home, and Cobb puts down his bags and spins his totem on the dining room table, before heading outside to play with his children. The totem continues to spin.
Just as Ariadne warns, Mal follows Cobb deep into the dream, and seems to sabotage the team's mission for good by killing Robert, sending him to "limbo." Ariadne's role in the conflict between Cobb and Mal is similar to the role that the Cretan princess Ariadne plays in Greek mythology in the conflict between Theseus and the Minotaur. Like the Ariadne of the Greek myth helps Theseus navigate Minos's labyrinth and slay the Minotaur, the Ariadne of the film helps Cobb descend into "limbo," rescue Saito, retrieve Robert, and confront Mal once and for all.
As the film's primary villain, Mal is an example of a femme fatale—an attractive or otherwise appealing woman who seduces the male protagonist in order to achieve her own nefarious ends. Nolan borrows the femme fatale trope from film noir, a hardboiled crime genre that became popular in postwar Hollywood for its seedy social realism, flawed protagonists, and suspenseful, expressionist style. Even Mal's own name derives from the French etymological root for the words "bad" or "badly"—e.g. malice, malcontent, maladjusted—which emphasizes that she has a negative psychological impact on the characters of the film, Cobb in particular.
The final act of the film takes place in what Nolan and his production crew called "Limbo City"—a ruined metropolis crumbling into the sea that broadly symbolizes the crumbling edifice of memories that Cobb is struggling to leave behind. Nolan pitched his vision of the city as, "something glacial, with clear modernist architecture, but with chunks of it breaking off into the sea like icebergs." The destruction of the city reflects the emotional turbulence Cobb feels not only in leaving Mal behind but in confessing to Ariadne that his guilt stems from the fact that the idea he "incepted" into Mal's mind seemingly caused her to commit suicide.
Whereas Ariadne pushes Cobb to realize that Mal is merely an artificial representation of her former self, the team succeeds in making Robert believe in the artificial illusion of his father, Maurice, as a man who was disappointed only in his son's attempts to be like him. The pinwheel that Robert finds in Maurice's safe is an item that, not unlike Charles Foster Kane's "Rosebud" sled in Citizen Kane, symbolizes a yearning for a life staked out according to one's own values, rather than one corrupted by the lure of wealth, fame, and power.
Cobb's successful attempt to retrieve Saito from limbo reveals that the opening scene of the film and the ending of the film are in fact intertwined, lending the film the structure of a Möbius strip. The last shot of the film, which lingers on Cobb's spinning totem, has prompted lively debate among critics and viewers about whether Nolan's intention was to suggest that Cobb is still dreaming, or whether he has finally returned to reality. Nolan himself has indicated that he designed the shot as a way to emphasize the story's "ambiguity" about whether the dividing line between reality and dreaming can ever be drawn with absolute certainty, leaving viewers to draw their own conclusions.