At the climax of "Fight Club,' the film's unnamed Narrator realizes that Tyler Durden (Brad Pitt) does not actually exist: in fact, he is Tyler Durden, and Tyler Durden is him. The Narrator experiences life as Tyler while the Narrator is sleeping, which is why characters throughout the film, like Marla, act so frequently concerned and frustrated about the Narrator's erratic behavior. As the Narrator races to foil Project Mayhem's plan to destroy buildings, Tyler confronts him, eventually holding the Narrator hostage with a gun as time runs out before explosives go off. However, the Narrator realizes that since Tyler is him, he is the one really holding the gun. The Narrator ultimately shoots into his own mouth to rid himself of Tyler's presence permanently. This leaves the Narrator with a gaping wound in his neck, even though Tyler is shown with a hole in the back of his head just before he dissipates.
"Fight Club" ends on an ambiguous note, with the Narrator insisting to Marla that he is finally feeling okay, even though he just shot himself and they stand in a skyscraper among buildings that Project Mayhem has just destroyed. The symbolism behind the Narrator ridding himself of Tyler is clear, but it's hard to reconcile the notion that the Narrator could shoot himself and live to tell the tale. It's also unclear how Tyler can die while the Narrator survives the gunshot, and the differences in wounds between the two is never accounted for.
