The novel takes places in a futuristic version of Chicago, Illinois, in which the population has been split into five factions: Abnegation, Amity, Candor, Dauntless, and Erudite. Each faction has a different “persona” and a different role in the city. Abnegation people are plain and modest, and go into governance; Amity people are kind and nurturing, and go into welfare; Candor people are honest, and make good lawyers; the Dauntless are brave, and work as soldiers and guards; the Erudite are intelligent, and study science and technology.
As the novel begins, Beatrice Prior—a 16-year-old girl living in Abnegation—is preparing for her Choosing Ceremony. At the age of 16, everyone in the city is made to take an aptitude test that determines what kind of person they are; i.e., which faction they belong to. Afterwards, the 16-year-olds attend a Choosing Ceremony, where they choose the faction to which they’ll belong for the rest of their lives. Beatrice worries that she’ll find out that she’s better suited for a faction other than Abnegation, thus disappointing her brother, Caleb Prior, and her parents, Andrew Prior and Natalie Prior. Beatrice is especially scared of disappointing her father, Andrew, an important government official, and Andrew’s friend Marcus, another influential government leader.
During her aptitude test, Beatrice is given a hallucinogenic serum. She sees a series of visions: a dog attacking a child, a man who interrogates her about her knowledge of a ”brutal murderer,” etc. At the end of the test, Beatrice’s examiner, a woman named Tori, tells Beatrice that she’s Divergent: a forbidden mental state that allows Beatrice to move between different factions. Tori makes Beatrice promise never to tell anyone about her Divergence—she warns that the government wants to kill off all Divergent people.
At the Choosing Ceremony, Beatrice sees her brother Caleb choose Erudite, instead of the usual Abnegation. Afterwards, Beatrice chooses Dauntless. Although she’s excited and curious about Dauntless, she’s also worried that her parents will be devastated by the loss of their two children: after 16-year-olds choose their faction, they’re almost never allowed to visit other factions.
Beatrice is taken to her new community in Dauntless. There, she meets the Dauntless leader, Max, as well as Eric, a young, frightening government official, and Four, a charismatic young man who trains new recruits. Beatrice renames herself “Tris,” and quickly makes friends with Dauntless recruits from other factions, such as Christina, Will, and Al.
The new recruits begin their training, competing for a small number of spots reserved for the most highly-ranked trainees. Four coaches Tris and her peers through combat exercises. Tris is a poor fighter—she’s no match for opponents like Peter, a huge, sadistic recruit. In her first fight with Peter, Peter sends her to the hospital. Afterwards, Tris resolves to train even harder. She distinguishes herself in a citywide game of capture-the-flag that’s designed to measure recruits’ abilities. Tris climbs to the top of the Chicago Ferris Wheel, allowing her team to surprise-attack its opponents. Tris’s bravery and quick thinking impress Four, as well as her fellow recruits. Four begins flirting with Tris—exciting Tris, but also making her a little uncomfortable.
Tris stands up for a weak-willed trainee named Al, offering to take his place in a sadistic punishment that Eric devises for him. At the same time, Tris begins to enjoy her combat training more and more—she doesn’t feel the same sense of mercy or caution that she was taught in Abnegation for so many years.
Tris gets a visit from her mother, Natalie Prior, and Natalie tells Tris that she knows about Tris’s Divergence. Natalie also tells Tris to ask Caleb to research serum when Tris next sees Caleb—Tris has no idea why Natalie makes this request. Slowly, Tris realizes that Natalie was Dauntless before she chose to live in Abnegation.
Eric releases a ranking of the new recruits: Tris is ranked low, while Peter is ranked second. That night, Peter stabs the top-ranked recruit, a boy named Edward, in the eye. Afterwards, Edward drops out of Dauntless. Then the recruits enter the second part of their training: psychological training. They’re injected with hallucinogenic serum and forced to experience the things that frighten them most. Privately, Four conducts a “fear simulation” with Tris, and discovers that Tris is good at resisting the effects of the serum. Four recognizes that Tris is Divergent, since Divergents can resist mental manipulation. Tris asks Four how he knows so much about Divergence, but Four doesn’t reply.
Reports begin to circulate about how Abnegation leaders—Andrew Prior, and his friend Marcus, in particular—have become corrupt. Although she’s infuriated by these reports, which she regards as propaganda, Tris concentrates on her training. She bonds with other recruits, such as Uriah and Marlene, who were born in Dauntless, rather than transferring from another faction. When the second round of rankings are released, Tris is at the top of her class. That night, Peter and a gang of followers try to kill Tris by throwing her into a chasm, but Four saves her life. Tris is shocked to discover that one of the people who tried to kill her was Al. Al, Tris realizes, has come to hate Tris for making him look weak. Tris angrily tells Al that she’ll kill him if he ever tries to talk to her again. Shortly afterwards, Al commits suicide by throwing himself into the chasm.
Tris proceeds with her psychological training. Four allows her to witness his own “fear landscape.” Tris notices that Four has only four fears—hence his nickname. One of Four’s fears is his father, Marcus, who abused him with a belt when he was a child. Four’s real name, Tris realizes, is Tobias—he used to live in Abnegation. Following the fear simulation, Tobias kisses Tris, and Tris kisses back. Tobias reveals that he knows Tris is Divergent, cementing their bond.
Tris fails badly at her own fear landscape, despite the early promise she’d shown. Frustrated with herself, she goes to visit Caleb, who’s now living in Erudite. Caleb tells Tris that he’s tempted to believe the things that Jeanine, his leader, had been writing about their father and about Abnegation.
Tris and her peers prepare for their final challenge: a fear simulation. In her fear simulation, Tris performs very well, mastering her own fears—including fear of having sex with Tobias. As a result of her good performance, Tris graduates at the top of the rankings. She and her peers are injected with a chemical, supposedly to allow Eric and Max to track their movements. Afterwards, Tris and Tobias have sex.
The next morning, Tris wakes up to find that her friends have been transformed into mindless Dauntless soldiers, thanks to the tracking chemical. Tris is immune to this mind control, however, due to her Divergence. She’s able to pretend to be a “zombie” as Eric and Max send their new soldiers into Abnegation. In Abnegation, Tris joins with Tobias—who’s also revealed to be Divergent, and therefore immune to the effects of the chemical. Soldiers capture Tris and Tobias and bring them to Jeanine, the mastermind behind the mind control scheme. Jeanine reveals her villainous plan—to use her new soldiers to conquer all of the city—then orders her soldiers to kill Tris and Tobias.
Tris is taken to a tank, where she’s to be drowned. Before this can happen, however, her mother saves her. Natalie sneaks Tris out of the building, and sends her to a secret compound where Andrew, Caleb, and Marcus are hiding out. Before she can leave with Tris, however, Natalie is shot and killed.
Tris joins up with Andrew, Caleb, and Marcus: they plan to break into the Dauntless headquarters and shut down the computers that control the soldiers. Together, they manage to infiltrate the compound and make their way to the computer room, but not before Andrew dies protecting Tris. In the control room, Tris finds herself facing Tobias, now controlled by a new, stronger mind control drug. Tobias raises a gun to Tris’s head, but Tris is able to convince Tobias to put the gun down by saying, “It’s me.” Freed from his mind control, Tobias switches off the computers, foiling Jeanine’s plot. The novel ends with Tobias declaring his love for Tris.