In the aftermath of the vicious Erudite-led assault on Abnegation, Jeanine's oppressive rule over Chicago intensifies as she declares martial law, blaming Divergents and their allies for the ensuing chaos. In the wake of destruction, Dauntless leaders Max and Eric stumble across a peculiar box adorned with the symbols of the five factions, believing it holds essential information from the city's founders that could resolve the issues surrounding Divergence. Consumed by her obsession, Jeanine seeks to capture every Divergent, convinced they possess the key to unlock the precious secrets within the box.
With the Abnegation stronghold left in ruins, a band of young rebels—including Tris (Shailene Woodley), her boyfriend Four (Theo James), her Erudite brother Caleb (Ansel Elgort), and the enigmatic Peter—seek refuge in the serene Amity compound, under the guidance of Johanna. Meanwhile, Eric leads a team to scrutinize the residents for Divergent symptoms. As trust is shattered when Peter betrays the group, they embark on a daring escape via a train into Factionless territory.
During their perilous journey, Four reveals his real identity as Tobias Eaton, the son of Evelyn, the leader of the Factionless. He opens up about the deep wounds of being abandoned by his mother and raised by his abusive father, Marcus. The Factionless tempt an alliance with the Dauntless to confront Jeanine's oppressive regime, yet Four chooses to carve his own destiny.
The following day, Tris, Four, and their companions head towards Candor, hoping to find other Dauntless survivors and solace in the faction known for its honesty. However, Caleb's disillusionment leads him to part ways with the group, casting a shadow over their journey.
Upon reaching Candor, Tris and Four face arrest by leader Jack Kang, who plans to hand them over to Jeanine. In a bid to clear their names, Four demands a trial by truth serum, which exposes the reality of their situation. Tris, overwhelmed with grief, confesses the tragic death of Christina's lover, Will.
As tensions rise, Erudite-backed Dauntless forces initiate a surprise attack on Candor, resulting in significant casualties. Max and Eric manage to recapture Tris, learning that her Divergent score is an astonishing 100%, making her the most valuable asset in uncovering the box's secrets.
When Four arrives to rescue Tris, he confronts Eric, executing him for his treachery. Back at Erudite HQ, Jeanine's frustration mounts as she struggles to unlock the box, driven to despair by the failure of her simulations. Meanwhile, Peter exploits the situation, maneuvering to manipulate Tris based on her selfless Abnegation backgrounds, leading to a climactic confrontation filled with danger.
As Tris and Four wrestle with the grim reality of war, they find themselves caught in a diabolical scheme at the Factionless base. In a moment of true desperation, Jeanine's cruel mind control disks are activated, compelling Christina (Christina Roberts), along with her friends Marlene and Hector, to advance toward a terrifying fate, pleading for Tris to surrender. Though Tori's quick action saves Christina and Hector, Marlene sadly meets her untimely end. Stricken by guilt, Tris resolves to comply with Jeanine's demands, willingly stepping into captivity.
At Erudite Headquarters, Tris stands defiant before Jeanine, refusing to partake in the simulations unless a cease of suicidal commands is granted. With Caleb and Peter observing closely, she bravely navigates the initial four trials, but exhaustion sets in, forcing her to rest. The devastating news of Four's capture during his rescue attempt spirals her into despair.
The following day brings chaos as Tris fails the Amity trial, with her vital signs comatose, shocking Jeanine and her cronies. Seizing the opportunity, Peter feigns Tris's demise to create further turmoil. Nevertheless, Four, uncovering the deception, breaks free from captivity with Peter's aid, determined to save his beloved.
More resolute than ever, Tris partners with Peter to decode the secrets within the box. Driven by a newfound clarity, she successfully opens it, unveiling a holographic message that upends their understanding—revealing the walled city and its faction system were merely an experiment with Divergents at its core; the world outside eagerly awaits their resurgence.
As Jeanine's power begins to crumble, she issues orders for Tris and Four's execution. However, the Factionless launch a brazen rescue, incapacitating Max and liberating the doomed duo. In the ensuing chaos, both Jeanine and Caleb are captured, while the message revealing the truth is broadcast throughout the city.
With her legacy firmly established, Tris yearns to explore the world beyond. Meanwhile, in the shadows, Evelyn whispers to Jeanine, alluding to a hidden existence beyond the walls—a captivating notion that tragically ends with Evelyn taking her own life.
