The Dauntless soldiers say nothing about why Tris and Tobias are under arrest. They lock them in a small room containing only a bench, and they are alone. Tobias says he can sense there is something bothering Tris—there is, because she has not yet confessed to him that she shot Will. He believes her when she says that it is just because she keeps thinking about her parents.
Some Candor and Dauntless come into the room, including Candor representative and leader Jack Kang. Jack says that they have been accused of crimes against humanity; he claims to have seen footage of them running the simulation in the Dauntless compound. This was Tobias under the influence of the simulation, and Tris trying to stop him. He does not give them time to explain; instead, he says the truth will come out when they are interrogated under the influence of truth serum. Tris hates the idea of truth serum, but she knows she cannot refuse.
Christina, Tris's best friend from Dauntless, comes into their holding room and she and Tris embrace, but inside Tris is worried of what Christina will do when Tris is under the influence of truth serum and admits to killing Will. Christina tells Tris that Will died, expecting her not to know, and Tris lies and says she saw him die on a monitor during the attack. On their way to the interrogation, Christina tells Tris that their other initiate friends—Uriah, Lynn, and Marlene—are all here too, except for Uriah's brother Zeke, who turned traitor.
Tobias receives the serum first. He tries to resist the serum but cannot, and reveals immediately that he is Marcus Eaton's son, and that he transferred from Abnegation to protect himself from his father. He reveals that he is Divergent, and that those who are Divergent can resist simulations—however, Jeanine gave him a Divergent-specific simulation that he could not resist. He describes exactly what happened in the Dauntless control room and how Tris was able to pull him out of the simulation. Finally, they ask him his biggest regret—he says that he regrets his decision to transfer to Dauntless, because he was meant to be in Abnegation and it was a cowardly decision. He admits that he was going to leave and become factionless before he met Tris.
It's Tris's turn, and she injects herself with the serum. She discusses her own faction transfer, and said she did it because she was too selfish for Abnegation. She explains how she was unaffected by the attack simulation. She manages to skip over the part where she kills Will until the very end, when they ask her her deepest regret. She admits that she killed him. Everyone in the room thanks her for her honesty, but Christina and Tobias, who did not know, remain silent.
Christina is extremely upset. Uriah finds Tris and tells her he understands, and that Christina will eventually. Tobias says they can talk about it tomorrow. Tris goes up to the roof of the Merciless Mart and screams, pitching a chair over the ledge and thinking about what it would be like if she threw herself over, too, just like the Dauntless initiate Al did in the pit. But she knows she cannot do that, because she needs to live in order to repay her parents for their sacrifice.
Tobias is more upset that she did not tell him in the first place, because he thinks she does not trust him. She accuses him of not trusting her either, because he did not tell her about Evelyn and his plans to ally with the factionless. After they fight she apologizes, but he seems to expect her to say more.
Later, Lynn comes and finds Tris, telling her to snap out of her stupor and start acting like she's Dauntless. She leads her to the large room that has become Dauntless's sleeping space. Tris meets Hector, Lynn's little brother, who is slightly afraid of her because of the rumors he has been told about the Divergent. Lynn says that she does not believe that Divergence exists—she thinks the leaders choose people at random and change the simulations for them as a different kind of mind control. Uriah and Marlene, who have been getting much closer lately, come by and inform them that a group will be going to the Hancock building for surveillance later that night. Tris agrees to go.
Lynn and Tris discuss why Lynn decided to shave her head—she wanted to appear as more of a threat to the Dauntless guys during initiation, who did not take her seriously. Tris thinks she could have used being underestimated to her advantage. Before they get outside the Merciless Mart, there are gunshots; Dauntless traitors have invaded. They loyal Dauntless fight back, and Tris is shot—however, it is not with a bullet. There are white cylinders around that release a smoke that knocks everyone unconscious except Tris, so she knows it must be simulation-inducing.
Tris decides to follow the traitors upstairs as they look for more people. She takes a dead traitor's blue armband that signifies the alliance with Erudite, so she can blend in. It turns out that Uriah is Divergent, too, because is awake as well. Uriah heads to the third floor to tell everyone to evacuate, and Tris goes to the second to look for more Divergent people who are not unconscious. She spots Eric, the ruthless Dauntless leader, dragging away a Divergent Candor woman who is awake. Tris begins stepping on pinky fingers to see if anyone responds; eventually a Candor girl does, and Tris shows her where to run.
Eventually, though, she realizes Eric has discovered her. They begin to fight—he reveals that he was the one who suggested to Jeanine putting Tris in a water tank to die. He captures her and leads her to the rest of the Dauntless traitors; Uriah has been captured too. At the last moment, though, Tris remembers that she has a knife in her back pocket.
The division of Dauntless into two groups, the loyal Dauntless and the traitor Dauntless, is a huge obstacle in the way of advancing the loyal Dauntless cause. That such a division can occur is an inherent flaw in the Dauntless nature, and Tris acknowledges this herself in Chapter 15 when the traitor Dauntless invade. She says that Dauntless is the only faction that would allow a split like this, and it shows the cruelty innate in them.
With Dauntless split in two, loyalty to either side will be ambiguous. Lines will become blurred as people wonder whether to side with their family and friends, or to side with what they believe is right. And, as Tris shows during the invasion, deception is as easy as putting on an arm band and pretending to be someone else. This stands in stark contrast to a faction like Amity, whose unity has strengthened them in the face of the war. Dauntless will not be able to emerge from this conflict powerful and strong if they remain divided.
No sooner does Christina come back into the picture than she is faced with a troubling inner conflict. Rationally, she must know that during a war entirely based on mind control, allegiances will shift, survival is the ultimate goal, and people will certainly die. However, her judgment is clouded by her love for Will and the betrayal she feels at Tris's actions. If Tris and Christina are to be friends again, Christina's head will need to reason with her heart. It becomes a question of how much she values their friendship.
Tris believed that telling everyone the truth about Will would clear the air for her and end her lies and deception. However, contrary to the Candor belief that the truth would set her free, the truth has only bound her even more by her guilt. Her confession under truth serum now has her wondering whether she was right to shoot him in the name of survival, or if she should have tried to find another solution as she did when facing Tobias in the control room. This is an uncomfortable yet immensely revealing question for readers to contemplate: would you have done the same thing in Tris's situation? Was she right to shoot her friend? Did she have another choice?
A rift has come between Tris and Tobias, too, as they realize that neither of them has trusted the other fully. It is important to note that they have come to this realization in the Candor compound, where honesty is the number one law. To have this confrontation occur here is a deliberate choice on Roth's part. In the black-and-white society that is Candor, they can no longer hide the truth from each other, and they must face the dishonesty that has come between them. This is an excellent example of the importance of setting: in literature, the setting often reflects the internal battles and emotions that the characters go through.
Tris briefly contemplates killing herself; this is a drastic change in pace from the rest of the story, because from the beginning of the attack simulation in Divergent up until this point, all of Tris's actions have been entirely focused on survival. But Tris only considers this for a second, and there are two reasons she decides against it. The first is the one she cites: she cannot let her parents down in this way, because they sacrificed themselves to keep her alive. The other internal and instinctual reason is that Tris does not know how to do anything but survive. Tris has been trying to survive throughout this war; she does not yet know how to die.
Now that Abnegation are sufficiently scattered or killed, the war waged by Erudite and the traitor Dauntless has become more of a war on the Divergent than anything. This is clear when the traitors invade not to attack and kill, but to find the Divergent and take them away. Neither Tris nor readers have a full grasp yet on why the Divergent are viewed as so dangerous and undesirable, but there are still many mysteries left to be solved. Expect the information Marcus mentioned, the information that Abnegation was trusted with and that Erudite are trying to steal, to have something to do with it.