Tris describes life in Abnegation and the rigid faction system. On Aptitude Test day, her results come back Divergent — inconclusive across multiple factions — and her administrator Tori warns her in secret that this is dangerous.

At the Choosing Ceremony, Tris transfers to Dauntless and her brother Caleb transfers to Erudite. Both Prior children leave Abnegation on the same day. Tris follows the Dauntless initiates as they run and jump onto a moving train.

The initiates arrive at Dauntless headquarters, jumping from a roof into a hidden net. Tris is the first to jump. Inside, she meets Four, who will be one of her trainers, and is renamed by her new faction.

Combat training begins. Tris struggles physically against initiates from Dauntless-born backgrounds who have trained their whole lives. She befriends Christina, Will, and Al, and begins to understand the brutal ranking system that will determine her future.

Phase Two of initiation begins: the fear simulations. Initiates are injected with a serum and placed inside realistic hallucinations designed to force them to face their deepest fears. Tris discovers she can think inside the simulation — manipulating it rather than simply enduring it. Four notices her anomalous behavior.

Four takes Tris into his personal fear landscape — a simulation containing only his four fears. She sees him as he truly is: Marcus's abused son, terrified of heights, enclosed spaces, killing, and his father. They acknowledge their feelings for each other.

Tris passes initiation. That night, all Dauntless members are injected with a tracking serum — actually Erudite's mind-control compound. The Dauntless become an army and march on Abnegation. Tris and Four are discovered to be immune. Four is captured and placed under a stronger serum. Tris fights through Dauntless soldiers, finds her mother — who was originally Dauntless — and watches her die saving her.

Tris reaches the Erudite control room where the simulation is being run. Four is there, partially controlled. She refuses to shoot him. He breaks through the simulation with her help. They destroy the program, stopping the massacre. Tris's father dies in the escape. The survivors board a train — Tris, Four, Caleb, Marcus, Peter — heading into an unknown future.