*SPOILER ALERT! This synopsis gives away major plot twists!**
The narrator, an average guy, develops a serious case of insomnia when his cubicle starts to feel like a prison and his possessions seem to, in fact, possess him. While the narrator is on his way home from a business trip, wondering if he'll every sleep again, his condo mysteriously explodes, reducing all the paraphernalia of his mail-order life to a pile of rubble.
Lost without the possessions that defined him, the sterile home that meant so much and so little at the same time, the narrator reluctantly picks up the phone and dials the only number in his wallet. Tyler Durden, an acquaintance of mere hours, represents everything this narrator longs for; he's powerful, strong-willed, and completely free of the claustrophobic cubicles of 9-5 workdays and the enslaving digits of a balanced checkbook. The two soon develop a brotherly bond, a rapidly deteriorating yin and yang, and together organize an underground group, a fight club.
The fight club revolves around the philosophy that a man can only be free when he's given up everything; that to bleed and to cause bloodshed is to embrace your own insignificance and that only by doing so can you ever truly live. Liberated from possessions, his cubicle, the trappings of morality, and the ideas handed down to him from father to son since the dawn of time, he now believes he's found something far more valuable -- himself. The narrator sleeps like a baby for the first time in a long time.
But as the fight club evolves, the narrator soon realizes that the rules and structure laid down so carefully by Tyler were created to protect him from the truth: He and Tyler are one person. He is Tyler. Could the narrator, this average guy, have invented Tyler in order to free himself from a life he hated, a self he hated? While he thought he was asleep, he'd actually been busy (or Tyler had been busy) organizing an operation of dreadful proportions, something far more sinister than bare-knuckle boxing and harmless pranks against corporations.
Now the narrator is challenged with the unimaginable -- how to prevent himself from crossing the line between liberation and mass destruction.