Seventeen-year-old Moonbeam has spent nearly her entire life inside the Base, the fenced desert compound of the Holy Church of the Lord's Legion, a religious sect in rural Texas. The novel alternates between two timelines, labeled BEFORE and AFTER, tracing the events leading up to a catastrophic armed confrontation and Moonbeam's painful recovery in its aftermath.
The story opens in the middle of the fire. Moonbeam sprints across the compound as government agents storm the Base with a tank and armored vehicles, gunfire tears through the air, and the Chapel burns. She races toward the western barracks, where children have been locked inside cabins. When she grabs the heated padlock on the first door, the metal sears off most of her palm. A government agent finds her, and she begs him to free the children, handing over a skeleton key. The children flood out. Moonbeam sees her fourteen-year-old friend Honey among them, alive. The last thing she hears before blacking out is Luke, a teenage Legion member, screaming to be allowed to Ascend, which in Legion belief means to die and join The Lord.
Moonbeam wakes in a hospital, disoriented and terrified, Father John's voice thundering inside her head, commanding her never to speak to Outsiders, the Legion's term for anyone beyond the community. Father John, born John Parson, is the Legion's self-proclaimed Prophet. She is sedated after panicking at the sight of a sheriff's badge, then transferred to a secure facility in Odessa, Texas, where she begins therapy with Doctor Robert Hernandez, a psychiatrist specializing in traumatized children.
Moonbeam refuses to speak at first but gradually begins exchanging questions with Doctor Hernandez. She asks about her mother, who was expelled from the Legion years earlier, and about an intercepted letter her mother tried to leave for her, but he has no information. During sessions, she compulsively draws the same image: a blue house with a white roof beside brown cliffs and a body of water, with two small figures outside, herself and her mother. In supervised group sessions with the surviving children, Luke appoints himself spiritual leader, preaching that their dead Brothers and Sisters have Ascended as Father John promised. Honey openly challenges him.
Through the BEFORE chapters, Moonbeam reveals the Legion's history. It was founded by a gentle preacher named Father Patrick, who welcomed visitors and allowed members to read books, watch television, and visit the nearby town of Layfield. John Parson arrived at the Base, spent three years building influence with the help of Amos, a senior member he won to his side, and staged a takeover. Father Patrick left without resistance, taking those still loyal to him. Father John issued Proclamations banning contact with the outside world, confiscating books and radios, locking members' rooms at night, and establishing four armed guards called Centurions as enforcers. Punishments included days-long fasts and confinement in metal shipping containers.
The Third Proclamation imposed celibacy on all men except Father John and ordered women to submit to him sexually. People who objected left. Moonbeam was designated one of his "Future Wives" at age ten, promised to marry him at eighteen.
Moonbeam describes the arrival of Nate Childress, a young man who becomes her closest friend and confidant. She also recounts catching Luke trying to force eleven-year-old Honey to touch him sexually. Moonbeam intervened, and Nate beat Luke severely. A week later, Moonbeam crept into Luke's room at night with her dead father's knives, held one to his throat, and warned him never to touch Honey again. Luke complied.
The turning point in Moonbeam's faith comes when her mother is tried in the Big House, Father John's residence and administrative center, for Apostasy. A secret journal revealing her mother's plans to escape with Moonbeam has been found. Her mother denies nothing, calling Father John "a snake-oil salesman" who preys on vulnerable people (280-281). She is found guilty and Banished. Father John forces Moonbeam to declare aloud, "I don't love my mother." Her mother hugs her one last time and whispers, "Under your pillow," but the letter has been intercepted.
Events accelerate. Horizon, the most senior Centurion, dies of lung cancer. Father John names Nate as his replacement, but Nate publicly refuses, saying he will accept only if The Lord tells him directly. That night, Nate wakes Moonbeam, gives her a cell phone for emergencies and a skeleton key that opens every door in the Base, and disappears. Father John declares Nate an Outsider spy and names Jacob Reynolds, a fanatical loyalist, as the new Centurion.
Father John orders all members to carry firearms and announces that God has chosen Honey, only fourteen, as his new wife. Honey refuses, and Father John orders her locked in a shipping container. Desperate, Moonbeam uses Nate's phone to call the Layton County Sheriff's Department, reporting a child locked in a box. She believes this call triggers the armed raid that follows, causing the deaths of 67 people. This guilt becomes the secret she carries through every therapy session.
In the AFTER timeline, Agent Andrew Carlyle of the Federal Bureau of Investigation (FBI) joins Moonbeam's sessions. Luke's mental state deteriorates until he dies by suicide, writing "I Ascend" on the floor in his blood. Doctor Hernandez asks Moonbeam, now the oldest survivor, to help stabilize the younger children. She agrees, seeing it as penance.
The critical revelation comes when Agent Carlyle tells Moonbeam her phone call did not cause the raid. The Legion had been under federal investigation for over two years after a package containing illegal firearm conversion kits and blasting caps, addressed to one of Father John's aliases, was intercepted. The warrants had been scheduled for service more than a month before her call; the timing was a coincidence. Agent Carlyle also confirms that Nate was an undercover agent of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Moonbeam's crushing guilt is lifted.
She eventually reveals what happened inside the Big House during the fire: the four Centurions lying dead in a circle, having shot themselves on Father John's orders. Father John tried to persuade Moonbeam to die with him. When she refused and called him a fraud, he insulted her and her mother. Moonbeam shot Father John in the chest and killed him. In the basement, she found confiscated personal belongings and, in the back of the bottom drawer of a filing cabinet, a letter addressed to her in her mother's handwriting.
Agent Carlyle returns the letter. Her mother writes that the Legion was a lie and that she spent years trying to escape with Moonbeam. She reveals the Third Proclamation was a fraud: most children born under it were fathered not by Father John but by men let into women's rooms by the Centurions at night. Pushing Moonbeam toward Father John as a Future Wife was the only way to keep her door locked and her body safe. Agent Carlyle also reveals that Nate's body was found in a shallow grave near the compound; he had been strangled, likely the night he tried to leave. The investigation confirms Father John was a convicted felon from California who attempted to overthrow another religious group before arriving at the Legion.
The surviving children gradually leave the facility for relatives or foster families. On Moonbeam's eighteenth birthday, Doctor Hernandez brings her to an empty room where her mother is waiting. Her mother explains she never contacted authorities because Father John threatened to have Moonbeam killed if she did. She developed a severe alcohol addiction, nearly died, and entered a recovery program in Oregon, where filtered news access kept her from learning about the fire until her release. The novel closes with Moonbeam sitting in the garden of her mother's small house in Portland, Oregon. There is no cliff, no water, and the walls are not blue, unlike the scene she drew compulsively for months. She reflects: "But it's okay. It's a start" (443).