One of the first scenes of the movie, Cobb and Arthur attempt to complete extraction on Saito through a dream within a dream. It is revealed that a person’s defense is lowered in the dream world, and thus, it is easier to obtain the person’s thoughts and secrets. They attempt to convince Saito that they are there to protect him when they are trying to get information about Saito’s business expansion plans from his subconscious. In the dream, Cobb encounters Mal even when she is not supposed to be there; she’s Cobb’s unconscious projection and later directly interferes with their extraction, causing their attempt to fail. 

In order for Cobb to return to America to his children, Cobb needs to perform inception for Saito, to help convince the heir of Saito’s main competitor, Robert Fischer, to dismantle his company. Cobb decides to attempt the inception, finding an architect named Ariadne to engineer the dream world layout for their inception. While in Mombassa, Cobb sees people who are addicted to dreaming. Their perception of reality and broken and they require special sedatives to remain in the dream world. 

The team kidnaps Fischer and are shot at by the defense of Fischer’s subconscious. Saito is wounded. It its revealed that the sedation used to stabilize the dream prevents him from waking up if he died. Instead, he will drop into limbo, unconstructed dream space and raw infinite subconscious. Nothing is left down there except by anyone sharing the dream who’s been trapped there before. People stay in limbo until the sedation wears off, which could take decades. Cobb explains that limbo becomes your reality, and you are lost down there for so long that you’re going to become an old man. 

The team kidnaps Fischer and are shot at by the defense of Fischer’s subconscious. Saito is wounded. It its revealed that the sedation used to stabilize the dream prevents him from waking up if he died. Instead, he will drop into limbo, unconstructed dream space and raw infinite subconscious. Nothing is left down there except by anyone sharing the dream who’s been trapped there before. People stay in limbo until the sedation wears off, which could take decades. Cobb explains that limbo becomes your reality, and you are lost down there for so long that you’re going to become an old man. 

Cobb attempts to turn Fischer against his own subconscious. Through deception, the team leads Robert to suspect that his Uncle Browning was trying to get him to follow his father’s footsteps and may not have his best interest in mind. 

Mal appears and Cobb hesitates to kill Mal, unable to tell if Mal is real or not, despite knowing that they are in an active mission 3 dreams down. Mal shoots Fischer. Cobb believes that they have failed and Fischer is already in limbo. Ariadne proposes they go another dream down and follow Fischer into limbo. Once Fischer is revived, he talks to his father who says that he was “disappointed that you tried” and finds a pinwheel Fischer made when he was younger. Fischer believes that his father not only cared about him but wanted him to build something of his own. 

Ariadne and Cobb follow Fischer into limbo, a world built by Mal and Cobb. There, Cobb reveals what happened between him and Mal. He releases his guilt and recognizes that he can’t stay with his projection of Mal in limbo and realizes Saito is dead by now. Fischer is revived and Cobb stays in Limbo to find Saito. 

The movie actually opens with this scene. Cobb is washed ashore on a beach. Armed guards drag him into a palace where Cobb meets Old Saito, seated at a dining table. 

Old Saito takes Cobb’s spinning top and spins it. The top wavers slightly in contrast to an expected perfect spin within limbo. Cobb tells Saito this word is not real, asks for him to come back. The two return back to reality. 

The team arrives in America and Saito makes a call, honoring his arrangement with Cobb. Cobb is able to enter the US with no issues with security. Cobb makes it back home and can’t believe if the world around him is real or not. He spins his top but before he can’t check if the top stops spinning, he sees his children’s faces, something that his memories have not shown him, either due to his guilt or because he is currently in reality. Cobb doesn’t stop to look at the top. He embraces his children. While the top wobbles, it never conclusively falls. The movie ends.