Resurgent: The Divergent Series (Book #4)

Tris is brought back from the brink of death and returned to the Chicago experiment which was successfully reset. Leading a new life, she once again faces her Choosing Day; new faction leaders have changed the Choosing age to 18. Will her choice lead her back to Tobias? Or will it launch her into a new life, back into the faction system they thought they would never see again?
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Towards the end of Allegiant, Tris is fatally wounded in an attempt to reset the Genetically Pure leaders of the Bureau of Genetic Welfare. Meanwhile, on a mission to stop the conflict in Chicago, Four finds himself trapped in the city; the Bureau had already found out their plan to stop them from the reset of the Chicago experiment.

Releasing a newly developed memory serum into the city, the reset becomes successful, affecting even Four. Because of Tris's importance as Genetically Pure, David makes every attempt to save her life. She is brought back from the brink of death.

The Bureau decides she was and will continue to be essential in the Chicago experiment. Tris is reset and sent back to the city.

Chicago is not what it used to be. New faction leaders are appointed and laws have changed, including a person's age of Choosing, which is now 18.