The novel is narrated by Lenny Winter, a mixed-race girl of Russian Jewish and African Christian heritage, who looks back roughly a decade later on the events of her junior year at an elite boarding school. She insists from the outset that, despite wishing a boy dead hundreds of times, she did not kill him.
The summer Lenny is 15, her mother, Lucretia, a community college sociology adjunct, and her stepfather, Yip, who was medically discharged from the army after a traumatic brain injury, move the family from Miami to Port Angeles, a small town on Washington's Olympic Peninsula, for financial reasons. They settle onto a houseboat called
Goodnight Moon. Lenny's biological father left the family years earlier; Yip, who married Lucretia when Lenny was two, is the only father Lenny has known. She befriends Sara Spaulding, a prickly girl her age living in foster care with her uncle.
In August, Lenny and Lucretia kayak to a remote cove called End of the World Beach to stargaze. Lucretia proposes they attempt a Messier marathon the following spring: a challenge to observe as many of the 110 catalogued deep-sky objects as possible in a single night. During the trip, Lucretia shows signs of confusion and slurred speech. Lenny, frustrated that her mother wants to leave early, argues with her and pushes her. On the return paddle, Lucretia loses consciousness. Lenny paddles back alone, but her mother dies of a stroke. Lenny is consumed by guilt, convinced her actions contributed to the death.
Sara warns that child protective services could send Lenny to her biological father if Yip lacks legal custody. Weeks later, Yip enrolls Lenny at the Blanchard School, a prestigious boarding school on a nearby island, on a scholarship. He says it honors Lucretia's wishes, but Lenny feels banished and accepts out of guilt.
The campus is Gothic, wealthy, and insular. Lenny befriends two fellow juniors: Vikram Flanagan, the chatty son of a Seattle tech CEO and an imports heiress, and Nico Soliz, a warm boy from Oakland with Bolivian roots. She is paired in woodshop with Henry Allen Quinn, a handsome senior whose father owns Scorpio Yachts. Henry proposes they build a canoe together. He is charming but controlling, and Lenny craves his approval.
The Pascalianum Club, or PC, is Blanchard's unofficial secret society, founded over 150 years ago by the school's original Patriarchs. It operates underground after being derecognized for hazing. Each year, senior members "tap" select juniors. Nico warns Lenny about the PC's ruthlessness, but she is drawn to the promise of belonging. In October, she and Vikram are tapped in a firelit ceremony in the woods, swearing oaths of secrecy and loyalty. Nico is not chosen.
Lenny's roommate, Sloan Calloway, a blunt senior dating Henry, lets slip that Yip may be giving up custody. Sara alerts Lenny to a student astronomy competition with a $10,000 prize for the best Messier marathon. Desperate for money, Lenny commits to the contest and steals a telescope from the physics lab with Henry's help.
Inside the PC, Lenny encounters a culture built on social currency: Members trade gossip as leverage and pay into a slush fund she can barely afford. She betrays a fellow scholarship student's secret to avoid covering a group bar tab. After the Día de los Muertos dance, she returns to find her room ransacked and her Adderall and Xanax prescriptions missing. Nico grows distant, hurt by his exclusion and Lenny's deepening involvement. The two share their first kiss in November, and their relationship deepens over the following months, but the PC drives a wedge between them.
In late March, the PC travels to Henry's parents' mansion in Seattle for Initiation night. The pledges are branded with a hot iron: Each receives a triangle on the forearm, but Henry gives Lenny an "inverse brand," a triangle scored with an X, marking her as different. She submits, sensing she has made a terrible mistake.
Lenny gets extremely drunk after the branding. In the early morning hours, she stumbles upon a cracked bedroom door and sees Henry on top of someone, his fingers inside her, while a phone on a tripod records. Sloan appears and demands Henry stop. Lenny blacks out. She wakes bleeding from the head; Nico finds her sobbing in the hallway and holds her until she falls asleep.
The next evening, Lenny, Nico, and Sara attempt the marathon but face repeated obstacles. They end up at Ediz Hook, a narrow sand spit near Port Angeles, where Vikram, Sloan, and Henry appear, having tracked Lenny via a location-sharing app the PC requires. Henry threatens to report the stolen telescope, then offers his father's speedboat. The group motors to End of the World Beach, where Lenny and Sara photograph deep-sky objects through the night.
On the beach, Sloan drunkenly confesses a series of truths. Vikram, not Nico, stole Lenny's drugs. Vikram and Henry have been scheming together since middle school. Henry orchestrated Lenny's PC recruitment from the start: She was chosen as the "diversity pledge" to serve as the annual "sacrifice" during Initiation. Sloan read Lenny's private journal and shared its contents with Henry, including the nickname "kitten." Then Sloan reveals the most devastating truth: At the Seattle house, it was Lenny, not Sloan, whom Henry sexually assaulted and filmed. Lenny's repressed memory returns. Henry was on top of her. She was barely conscious. Sloan walked in and demanded he stop.
Nico, hidden behind a boulder, overhears everything and storms toward the group. Henry refuses to delete the video and plays it on his phone, mocking Lenny with racist language. Lenny attacks him. Henry punches and kicks her. Sara grabs the telescope and strikes Henry in the back of the skull. He dies.
Sloan takes charge. They disguise the wound, delete the video from Henry's phone, load his body onto the speedboat, and crash the boat into the rocks at Ediz Hook to stage an accident. Lenny breaks her arm in the crash and wakes in the ocean beside Henry's mangled body. She briefly considers drowning, then swims to shore.
Detective Thibeaut investigates but faces political pressure from Blanchard alumni. The coroner rules Henry's death an accident. The woodshop teacher, Murchie, a Blanchard alumna, later reveals to Lenny that she too was once the PC's sacrifice and bears the same inverse brand. The school's board of trustees, populated by former PC members, has long tolerated the society. Lenny recognizes that Henry's assault, though it exceeded prior traditions, was enabled by conditions embedded in the institution.
Lenny discovers that Erika Maines, the woman Sara believed was Yip's girlfriend, is actually a court-appointed social worker. Yip has been fighting for custody all along; Lenny's biological father formally renounced parental responsibility. When Lenny confesses that she believes she caused her mother's death, Yip tells her that is not how strokes work. He asks if she wants to come home. She says yes.
In the novel's final pages, Lenny writes from
Goodnight Moon's top deck, preparing for Yip's fifty-fifth birthday party. Sara married and returned to Port Angeles. Sloan works in Los Angeles. Vikram works at a Bay Area start-up. Nico and Lenny broke up after high school but remain close. Lenny attended woodworking school. Their marathon entry placed second, winning $5,000. All five survivors met once at a riverside cabin, where the three women held hands around a dying fire and Lenny told Sloan she forgave her. Lenny closes by addressing her mother, affirming that life has meaning precisely because it ends.