Eric announces that his orders are to take only two Divergent back to Erudite headquarters for testing, and execute the rest. He begins to execute the younger people, claiming their brains are not yet mature so their Divergence is not fully developed. He says the same of Tris, but as he speaks to her, she lurches forward and stabs her knife into his heart. A fight begins, but luckily loyal Dauntless stream in at the right moment and fire at the traitors. Since the traitors weren't prepared for an attack, most of them run. For now, Eric is still alive.
Later, Tobias tells Tris exactly what happened: one of the Erudite citizens sent upstairs to search for Divergent actually went up much higher and warned and evacuated the loyal Dauntless, who organized the ambush in the lobby. This Erudite was Cara, Will's older sister.
Tris examines what was shot into her skin during the first attack; it is a metal disc with a needle on it, injecting some sort of blue dye into her skin. She pries it out, wondering what it is. Uriah finds Tris and reports that Eric is still not dead, and some Candor decided to give him medical treatment so that Jack Kang could interrogate him. They try to figure out why Jeanine wants to kill the Divergent if she already has an army. Tris thinks she must be planning another simulation. Before they head back to the rest of the group, Uriah asks Tris not to tell anyone he is Divergent.
Tris spends the night removing needles from everyone's arms. In the morning, she is surprised to see Caleb arrive—he has come to the Candor compound with Marcus, who apparently escaped the shooting in the Amity compound with Peter. Peter has turned traitor and gone to Erudite. Tris sees Tobias later, and knows she has to tell him that Marcus is here—however, before she can, other people's taunts in his direction bring it to his attention first.
Together they reason through Jeanine's attack. Tris shows him the puncture would on her arm, and the patch of blue dye that has still not yet faded. Tris's theory is that they are developing a long-lasting simulation transmitter, and that is what they shot them with; that way, they will be vulnerable to multiple simulations. As Tris describes everything else she did during the attack, Tobias is frustrated that she would throw herself into a situation like that with no gun when she could have just run away.
Jack Kang addresses everyone after he has gained as much information as he can. He says that the Divergent seem to require more investigation. He asks all the Divergent to reveal themselves. To everyone's surprise, Marcus is one of the ones who steps forward. He insists that the Divergent are not dangerous. Tris and Tobias speak up and say that the Erudite intend to kill the Divergent. Jack calls the invasion of the traitor Dauntless and the Erudite peaceful, and says he hopes to be able to negotiate a peace treaty with Jeanine. Tris insists that the invasion was not peaceful, and tells them what she thinks the needles were for. Jack accuses Tris of being stressed and confused and unable to think objectively. The loyal Dauntless all want to attack Erudite, but they cannot do it without Candor's help. Tris thinks they need the factionless.
One day a traitor Dauntless arrives at the compound, pulling along an injured woman. They are Tori and Zeke, respectively. Tris insists that they take her up to find a Dauntless nurse to give her medical attention. Zeke explains that the traitor Dauntless found out the two of them were collecting information from them, and shot Tori as they tried to get away. Uriah comes to greet his brother—apparently he and Shauna were the only people who knew they were spies. Zeke is cleared in a Candor interrogation under truth serum. He says he was mostly working on getting Erudite defectors out safely, but that Tori was doing something secret that he did not know about.
Christina and Tris still have not spoken, and Tris watches apprehensively as Cara, Will's sister, approaches Christina at a table in the gathering room and begins talking to her. Tris follows them out of the room to listen to their conversation. Christina cries and says she cannot stop thinking about what Tris did, but Cara, always the Erudite, reasons logically and explains that Tris did the only thing she could in a difficult situation. She tells Christina she does not have to forgive Tris, but that she should try and understand why she did it. She takes a few rude jabs at Tris, because apparently she never liked her in the first place, but Tris is nevertheless grateful for what she said.
Tobias revelas that Jack Kang will be meeting with a representative of Jeanine's the next morning. They try to figure out what Erudite might say at this meeting, and they look towards Tris for an answer since she has aptitude for Erudite. Shauna, since she is suspicious of the Divergent, questions Tris's loyalty and storms out. Tris is annoyed, but tries to reason. She said she thinks Jeanine will try to manipulate Jack, and she thinks Jack will do anything he can to protect his faction, even if it means sacrificing the Divergent. They decide they need to listen into this meeting.
That night while they sit and eat dinner, a bunch of Dauntless mock Tobias for his cowardly confession while under truth serum. Tobias is fed up with this treatment and decides to do something about it. He goes over to where Marcus is sitting, wrestles him from his seat, and punches him in the jaw. He drags him in the center of the room and begins to whip him with his belt, just like his father used to do to him. Tris goes and pulls him away, and can see in his eyes that he has been planning this for a long time. Lastly, Tobias pulls his mother's wedding band from his pocket and throws it at his father.
A Candor man walks Marcus to the bathroom, and Tris follows. She reminds Marcus about her offer to help him get back whatever he wants from Jeanine, and tells him to come find her when he has gotten over his delusion that she is useless.
It is interesting to juxtapose Tris's reaction to shooting Will with her reaction to stabbing Eric. Stabbing requires much closer and direct contact with the target; she cannot simply close her eyes and shoot from a distance. However, she is much more affected by what she did to Will than what she did to Eric. Tobias makes a much larger deal of Tris's stabbing Eric than Tris herself does; Tris moves past it very quickly. This reveals something about Tris's character: it is not the act of killing, attacking, or injuring that she has a problem with. Tris is fully capable of killing intentionally. It is only when someone she loves stands on the other side of the gun or the knife that she feels the guilt of committing murder.
In these chapters, a lot of characters presumed to be traitors return to the loyal side: Cara, Tori, Zeke. Conversely, though, others move to the Erudite side, notably Peter. These transfers of loyalty bring out the theme of betrayal present in this novel. During war, it is difficult to pinpoint exactly where a person's loyalty lies, and constant new information and events can make this loyalty ambiguous. As such, betrayal is common, and Tris and her following must learn to either cope with this truth or use it to their advantage.
Piece by piece, Tris and the others are slowly putting together Jeanine's plan. It means grave danger for the Divergent, and has begun to create an additional division on the loyal side between the Divergent and the non-Divergent, motivated by fear and uncertainty. So far, this war has done nothing but divide the factions into different groups and pit them against each other: first Erudite against Abnegation, then a split Dauntless, and now an inner divide among the Divergent. One of the author's most important messages in Insurgent is that divisions can destroy.
Tris has slowly been stepping into a leadership role among her friends and the loyal Dauntless, which is a new development in this book because for the majority of Divergent, she was just an initiate following orders. The others look to her to reason Erudite's point of view, to stand up for what she believes is right, and advocate for the rest of them. This is extremely important in Tris's development as a character; she is gaining some essential confidence. However, it also means there is huge responsibility placed on her shoulders.
At the same time, though, Tris's relationship with Tobias is under a lot of strain. Both have been preoccupied with their other responsibilities, and there is still some tension in the air from their argument over the lack of trust between them. Tris has stated before that she does not want to use Tobias as merely a crutch to make her feel better when she is under a lot of stress. As a result of this resolution, however, they have not made a lot of time for each other outside of their struggles.
Tobias himself has been much more secretive lately, involved in trying to glean as much information from the faction leaders as he can. The cold, calculated episode in which he beats up his father shows that he has been thinking thoughts and brooding over things that he has not been sharing with Tris. She can tell that this was a planned assault, not something done in the heat of the moment, and this proves that there are things he is keeping from her as well.
Meanwhile, Tris is on the brink of a deal to help Marcus get whatever information he needs from Abnegation. Remember, when Tris told Tobias about this information, Tobias denied that it existed and thought his father just wanted to seem more important than he really is—this means that Tris will be alone in this pursuit, since she does not have Tobias's support. The exchange at the end of Chapter 20 foreshadows a relationship between her and Marcus based on mutual need. She loathes Marcus nearly as much as Tobias does, but she knows that whatever information he has knowledge of could be key to their survival. She knows she needs him, and he needs her.