Far into the future, the United States has been rocked by floods and a volcanic eruption that caused massive destruction. In the wake of these apocalyptic events, a new Republic has formed. This Republic is a totalitarian state run by a dictator named Elector Primo. A dissenting faction, the Colonies, has also formed, which resists the Republic’s rule. Day is the Republic’s most wanted criminal. He is a fifteen-year-old boy from the poor Lake sector outside of Los Angeles. After failing the Republic’s “Trial,” a grueling test every child takes at the age of ten which determines their educational and economic future, and being subjected to painful medical tests while in a labor camp, Day decides to live a life of crime against the state. He is known for torching fighter jets, vandalizing public buildings, and stealing from banks. Day, only fifteen years old, is very clever and has managed to elude capture for several years—the Republic doesn’t even know what Day looks like. Even though it would be safer for Day to stay outside the city, he remains close to his family’s home to keep watch over his two brothers, John and Eden, and their mother. On the other side of town is June, a fifteen-year-old prodigy who was born to wealthy parents. She is the only student known to have gotten a perfect 1500 on the Trial. June, cocky and self-assured, awaits graduation from Drake, one of the nation’s top schools. She’s in line to become one of the Republic’s premier citizens. 

One day, June learns that her brother, Metias, a military captain for the Republic, has been murdered. Metias’s friend and military colleague, Thomas, comes to break the news to June. He tells her that Metias was murdered during a break-in at a local hospital while during rounds. Everyone suspects Day is the killer since he was the one who committed the break-in. Day was searching for plague cures for his family. Cures are expensive and inaccessible to the poor, who have to pay for medical care, unlike the wealthy, who receive free care. When June arrives at the scene of her brother’s murder, Commander Jameson, a hardline official, informs June that she’s being promoted to officer right on the spot, and her first assignment will be to investigate her brother’s murder. June swallows her emotions, puts on a brave face, and stoically assumes the role. After examining her brother’s body, however, she vows to avenge his death and hunt down his murderer. Meanwhile, Day, who managed to escape the hospital with only an injured leg, doesn’t know if he killed Metias. Day, who is known for not murdering his hostages, only recalls throwing the knife at a soldier, being careful to not kill him. Day is nursed back to health by Tess, a younger orphan he took under his wing when he found her scavenging for food on the streets. Day and Tess take care of each other.

June goes undercover to find Day, whom she is convinced killed her brother. While undercover, she stumbles upon a Skiz fight, a type of street fight where gamblers take bets on random fighters. June is inadvertently brought into the ring when she intervenes to help Tess, who is challenged to a fight. Although June doesn’t know Tess, she sees that Tess is too thin and fragile to compete, and could easily get killed. When June wins the fight but refuses to pick another opponent, the angry gamblers chase after her. Day, who is waiting for Tess outside the ring, throws a dust bomb to disperse the crowd. Day decides to take June in, and he, Tess, and June become a unit wandering the streets together. Both Day and June are clever, intelligent, and perceptive. They try to hide their identities from each other because they don’t know who the other one is. After a few days, however, June pieces together that the boy who helped her is Day. She struggles with this realization, however, because Day has been so caring and protective of her. They’ve even grown attracted to each other.

Despite her attraction to Day, and her instinct that he is a good, caring person, June summons up the resolve to turn Day in. She alerts Thomas, who stages a capture at Day’s family’s home. June tells Day that the plague patrol is coming to his house to take a sick family member away. Day races toward his family’s home. He rushes inside the house to bring them to safety, shocking his mother, who believes he died in the labor camps years prior. It’s too late, however. The soldiers come and outnumber him. Day starts sling-shotting bullets at the soldiers out of desperation. Commander Jameson, impatient to draw Day out, orders Thomas to shoot Day’s mother on the spot. Destroyed and distraught, Day and the rest of his family are captured and brought to the military’s headquarters.

June is put in charge of guarding and interrogating Day, who is sentenced to be executed. The more she questions him, however, the more she begins to doubt that Day is the one who killed her brother. Day tells her that the Republic doesn’t send kids to labor camps but kills them outright when they fail their Trials. He also tells her he suspects the Republic deliberately infects the poor sectors with the plague. One night, June reads her brother Metias’s journals. She remembers that the night before he died, he told her he needed to tell her something. In the journals, June finds a code to a secret website her brother created to give her the shocking news he found out: Their parents, whom they believe had died in a car crash, were murdered by the Republic, and Thomas found out that Metias knew about it. June realizes that it was Thomas who killed Metias, not Day, and the Republic is trying to cover it up.

June rushes back to Day’s cell. When Day reiterates that he didn’t kill her brother, she accepts his innocence and resolves to save him. June enlists the help of the Colonies’ rebel soldiers, the Patriots, who create a diversion that allows June, Day, and Day’s brothers—John and Eden—to escape. The ruse doesn’t work as planned, however. To buy them more time, John makes a fateful decision to switch himself with Day. Since Day is barely conscious after being beaten, Day isn’t able to stop him. John wears a hood and walks onto a stage where the Republic executes him. When Day comes to full consciousness, June assures him that his brother John wanted to sacrifice himself. Day, anguished, resolves to save both his brother Eden, who he now knows was deliberately infected with a new strain of the plague, and Tess, who has gone undercover with the Patriots. June, who now realizes Day is her only family, commits to helping him.