Allegiant, the third book in the Divergent Series by Veronia Roth, opens up with Tris imprisoned for her traitorous actions in the previous book. While she is imprisoned, Tobias is debating what he wants to do. While he wants to support his mother, Evelyn, he also wants to leave the city. Tobias hears that Tris is going to be interrogated, but he knows that she will be able to resist the truth serum and come up with a lie to save not only herself, but Christina and Cara, who were also imprisoned with Tris. Tris lies and says she was manipulated into following orders from Marcus, believing him to have been working with Evelyn. The lie works and Tris and the others are released from prison.

Things in the city are shown to be very chaotic. Riots are breaking out, with the rioters calling for the death of factions. At the end of one riot, Edward is killed. This makes Tobias realize that Evelyn is at fault for taking away everyone’s choice. As a result of the riots, stricter rules are put into effect throughout the city. In the aftermath of the riot, Tris is approached by a rebel group known as Allegiant. Allegiant wants to help Tris and her accomplices escape the city. Before leaving, Tobias rescues Caleb who was going to shortly be placed on trial. The two escape along with Tris and her group.

Everything soon turns chaotic and the group is attacked by Dauntless after leaving the city. Tori is killed in the attack, but Tris manages to escape and reconvene with Christina and Johanna, who inform her about the fate of the others. Tobias and several others are missing, but the group manages to get back together, with Tobias having suffered a gunshot to the arm. The group begins to explore the new landscape.

While exploring, the group meets a woman named Zoe, and much to Tobias’s surprise, find her with his former trainer, Amar. Tobias is naturally surprised, since he believed Amar to be dead. Zoe tells them that she and Amar work for a group that was responsible for founding the city. Tris decides they can trust them and they are led to the Bureau of Genetic Welfare in what used to be O’Hare Airport. Zoe takes the group inside to meet their leader, David, someone Tris recognizes from a picture he was in with her mother.

David explains that their group used to be part of the failed United States Government. He tells them that the former government failed because they tried to weed out negative traits and emotions in their citizens. Obviously not everyone agreed with this which led to a huge civil war. After the war, all the genetically damaged survivors were put into cities where they bred and created genetically healed humans, the Divergent. David and his Bureau have been studying the surviving societies.

Everyone takes some time to process this information, with Tobias and Amar briefly reconnecting to discuss why he faked his death. Tris is given documents about her mother, discovering that her genes were considered nearly perfect. Tobias and Tris get their genes tested, with Tris discovering that she is Divergent and Tobias isn’t. Tobias is hurt by this, but Tris seems him as the same person. The Bureau flies Tris and the others out of the airport. Tris studies some of her mother’s journal, while Tobias finds out that his father has been exiled from the city. That evening, Tobias is surprised by Nita, who sneaks him out of the Bureau to show him the fringe where the genetically damaged live. She tells Tobias the government only makes their lives worse, not better.

Tobias tells Tris about his meeting and takes her to meet Nita. Nita wants them to help her steal a memory serum that the Bureau supposedly wants to use to reset the memories of everyone in Chicago. Tris doesn’t believe Nita, but Tobias decides that he wants to help her. Tris ends up being right and Nita sets off a bomb and takes David hostage. Tris learns that Nita has stolen a death serum and is using David as a human shield to escape. Nita and Tobias are stopped and arrested as traitors, although Tobias is soon set free.

While Nita was acting on her own accord, she hadn’t lied about what the Bureau had planned. Tobias heads off to the city, wanting to inoculate some of the people still inside. In the city, Tobias manages to convince Evelyn to let everyone in the city be free and form an alliance with Allegiant. While that is going on, Tris tries to stop the Bureau from using the memory serum, deciding to inflict it on them instead.

Even though it is a suicide mission, Tris unleashes the memory serum on the Bureau. Even though she succeeded, David was still able to shoot her as the serum is being released. Tobias returns and mourns the loss of Tris, deciding to wipe his own memory. He’s talked out of this though by Christiania. In the epilogue, Tobias is still sad about the loss of Tris, but he’s determined to help everyone heal and turn the world into a better place.