The student body of Opportunity High School in Opportunity, Alabama, prepares to return to school after the winter vacation. The novel’s four narrators – Claire, Tomas, Autumn, and Sylvia – each grapple with different fears as the start-of-term assembly begins. Immersed in track practice outside the school, Claire wonders what she will do after graduation and worries about being separated from her best friend and running partner, Chris. Grieving the second anniversary of her mother’s death in a car accident, Autumn feels alienated from everyone around her except her secret girlfriend, Sylvia; she’s worried about her older brother Tyler, a former drop-out who recently reenrolled in school but is nowhere to be found as the students file into the auditorium for the assembly. Meanwhile, Sylvia has just been accepted into Brown but is unsure if she should go; she fears being separated from Autumn and also feels responsible for her Mamá, who is currently dying from a degenerative illness. Tomás, Sylvia’s twin brother, has skipped the assembly and is rooting through student files in the principal’s office with his best friend, Fareed; he’s looking for information on Tyler, whom he believes has been harassing his sister and is responsible for her withdrawal from him and the decline of their formerly close relationship.
Principal Trenton repeats the predictable speech she gives every year, telling the students to work hard and develop good character, and afterwards students start filing towards the back of the auditorium. Despite her dismal mood Autumn chats with an acquaintance about their mutual love of dance and her desire to audition for Julliard. However, she and Sylvia soon realize there’s some sort of holdup in the back of the room; just as students start whispering that the doors are locked, Autumn’s brother Tyler enters the front of the auditorium, brandishing a gun.
Standing in the principal’s office, Tomás and Fareed wonder why students coming from the assembly aren’t filling the halls; when they hear two shots ring out, they immediately understand what’s going on. Fareed calls the police, who instruct them to leave the school through a window and wait for help outside. However, Tomás knows that Sylvia is in the auditorium and refuses to leave her there. The two boys decide to stay and run to the janitor’s office to arm themselves with tools against the shooter; there, they find that the kindly janitor, Neil, has been strangled.
Outside, Claire and her track teammates hear the shots as well. Coach Lindt is overcome by shock and incapable of giving instructions, so it falls to Claire to decide what to do. She instructs some of the younger girls to search for a phone near the school while she and Chris run for help. Before leaving the school they look for the security guard, Jonah, only to discover his dead body next to his car. Nearby Claire recognizes a car belonging to Tyler, her ex-boyfriend; she’s horrified to discover that the backseat is full of gun ammunition, and realizes that he must be the shooter.
Sylvia and Autumn are near the back of the auditorium, but they have a clear view of Tyler shooting at teachers as they try to confront him. The first to fall is Principal Trenton. Autumn, who has always relied on her brother to protect her from her abusive father and support her passion for dance, is shocked that he’s capable of murder. On the other hand, Sylvia knows that Tyler is capable of violence. In a flashback, she remembers an afternoon last summer when he confronted her about her secret relationship with Autumn. Blaming her for “corrupting” his sister, Tyler raped her in “punishment,” but Sylvia has never told anyone about this.
Climbing onto the stage, Tyler tells everyone that they’re under his control now, clearly reveling in the fear he’s inspired. Ever since his mother’s death and father’s descent into alcoholism, Tyler’s behavior has grown more erratic and aggressive; he’s gotten into so many fights with other students, especially Tomás, that he had to drop out of school. Autumn is the only person who loves him despite his flaws and hopes he can turn his life around.
Claire and Chris run away from the school, hoping to reach a gas station and call the police. Claire is worried about her younger brother, Matt, who is inside the auditorium. She also feels guilty that, although she was Tyler’s girlfriend, she never anticipated his actions, even though she saw him becoming more and more angry. She even witnessed him threaten Sylvia at a school dance, an event which precipitated their breakup, but didn’t tell anyone about it. However, Chris comforts her and reminds her that she doesn’t have any responsibility for Tyler’s actions.
In the auditorium, Tyler starts accuses the community at large of abandoning him and turning his own family against him. Sylvia fears that Tyler has come for revenge against her and that she’ll soon be targeted. As Tyler moves through the front rows, finding students with whom he has particular grievances and killing them, Autumn crawls slowly towards her brother. In a flashback, she remembers his anger when he discovered her plans to apply to Julliard, which he sees as an act of abandonment. In fury he exposed her to their Dad, who has forbidden her to dance and beats her as punishment. Tyler demands that his sister show herself, and after a long pause, Autumn gathers the strength to stand up.
With bolt clippers from the janitor’s office Fareed heads off to cut the chains around the front doors, while Tomás takes paperclips to the locks near the auditorium. However he fails to pick the locks and must wait for Fareed to return. In the meantime, he taps out a soft rhythm – a favorite childhood lullaby – on the door. When someone returns the rhythm from the other side, he realizes that he’s right next to his sister. When Fareed arrives they use the bolt cutters to snap the chains around the doors and carefully open them. Tyler’s attention is distracted by Autumn, who is onstage trying to explain to Tyler that she loves him, and it’s in fact he who alienated himself from family and friends. Taking advantage of the distraction, Sylvia, Fareed, and Tomás crawl among the terrified students and silently urge them to escape.
Claire and Chris encounter the police, who have already been alerted by Fareed’s call and several students using their phones from inside the auditorium. In fact, lots of students are tweeting to the outside world and communicating with loved ones about their fears. They ride back to school in the police cruiser, but the officers won’t give them any information about the shooting or accept their frantic offers to help. They stand outside the school and watch helplessly as parents begin to arrive and demand information about their children. In the midst of all the confusion, Chris admits that he has long had feelings for Claire and is “terrified” of losing her; they kiss, and Claire suddenly feels that her future is less alone and uncertain.
Autumn continues to talk to her brother, knowing she needs to hold his attention while students sneak out of the auditorium. Accusing her of valuing dance more than him, Tyler forces her to dance on the stage, which she does; but suddenly he kicks her to the ground and hits her several times. He starts shooting at Sylvia, who flees the auditorium with Tomás and Fareed. They decide to hide upstairs, even though Sylvia fears that Tyler will never leave the building while she’s inside. Tyler exits the auditorium through another door. Left inside, Autumn tries to care for Claire’s younger brother Matt, who has been wounded.
Upstairs Sylvia, Tomás, and Fareed hear Tyler mounting the stairs. They find an unlocked classroom and use its window to get onto the roof. Sylvia and Tomás hug and apologize to each other for being so distant in the past months, but at the last minute Tomás jumps back into the classroom to distract Tyler, knowing that he can give Sylvia some time to escape even if it means sacrificing himself. He encounters Tyler in the hallway, and after a brief confrontation Tyler shoots him and kills him. Sylvia struggles to open the heavy window; just as she descends back into the classroom, she hears shots and knows that her brother is dead.
Autumn finds a phone so Matt can call his sister, Claire. He pretends to be unharmed, but Autumn can see that he’s growing weaker from his gunshot wound and will probably die. SWAT officers empty the auditorium and round up the uninjured students, but despite Autumn’s desperate pleas for help they say that paramedics can’t enter the building until it’s secure. She’s forced to leave the auditorium and Matt stays on the phone with his distraught sister until he dies. Chris holds Claire as she sobs.
Autumn manages to sneak away from the group of escaping students and runs up the stairs, searching for her brother and Sylvia. She finds Tyler standing over Tomás’s dead body and ranting about “showing Sylvia her place.” Again, she tells him that she loves him and tries to convince him to step down, but it’s impossible to change his feelings. He spitefully shoots her in the knee, thus ending her aspirations as a dancer, and then kills himself.
The police storm up the stairs and enter the classroom where Sylvia and Fareed are hiding. Sylvia walks out in the hallway to see Tyler and Tomás lying dead on the floor and Autumn writhing in pain and clutching her brother’s hand. Despite her grief for her brother, Sylvia knows she must stay strong and calm for Autumn, and she soothes her girlfriend as paramedics load her into an ambulance.
That night, after the police have finally left the school, Fareed breaks onto the roof and organizes a ceremony of remembrance. All the students stand in a circle and light paper lanterns inscribed with the names of the dead. It’s a nod to a similar yearly ritual organized for seniors by the beloved English teacher Mr. Jameson, who has also died at Tyler’s hands. Fareed urges the gathered crowd to live good lives in honor of the friends and relatives they have lost. Sylvia holds her brother’s lantern, wanting to keep him close as long as possible; but knowing that she needs to release him “toward the promise of a new day,” she finally lets go.
