The central twist of "Fight Club" is the revelation that the Narrator and Tyler Durden are two personalities inhabiting the same body; Tyler is a creation of the Narrator's insomniac mind. This re-contextualizes the entire film. Every scene where Tyler interacts with another character is actually the Narrator interacting with them. When the Narrator fights Tyler, he is fighting himself. Tyler's sexual relationship with Marla is, in fact, the Narrator's. The film is filled with clues to this twist: Tyler's subliminal flashes before he's introduced, moments where other characters react strangely to the Narrator when he thinks he's with Tyler, and a scene where the Narrator gets off a bus he just saw Tyler board.
The creation of Tyler is the Narrator's desperate attempt to break free from his own perceived weakness and the constraints of his consumerist life. Tyler embodies the id—the primal, impulsive part of the psyche. The final act sees the Narrator realizing he has lost control of his alter ego, as Project Mayhem plans to blow up buildings housing credit card records. The climax occurs in one of these buildings, where the Narrator fights Tyler for control of their body and mind. Recognizing that he and Tyler share the same body, the Narrator realizes he has the gun, not Tyler. To rid himself of his destructive alter ego, he shoots himself through the cheek. This act of willpower, not the bullet itself, kills Tyler. In the final scene, the Narrator, now seemingly in control, holds hands with Marla as they watch the buildings explode, an ambiguous ending that suggests a new, uncertain beginning free from both consumer debt and the tyranny of his alter ego.