Five years before the story begins, the town of Sunnybrook lost five high school cheerleaders in three separate incidents over two months. Colleen Coughlin and Bethany Steiger died in a car crash on a rainy night. Weeks later, Juliana Ruiz and Susan Berry were strangled inside Susan's home. Their neighbor Jack Canning was identified as the suspect, and Sunnybrook police officer Tom Carlino shot and killed him. The fifth cheerleader, Jennifer Rayburn, had been home with strep throat the night of the murders and took her own life shortly after.
Monica Rayburn, Jen's 16-year-old sister, is now starting her junior year. She lives with her mother Phoebe, her stepfather Tom, now a sergeant in the Sunnybrook Police Department, and her 10-year-old half brother Petey in the Waverly Estates development. Monica is secretly recovering from a medical abortion, arranged with her mother's help and hidden from Tom. The pregnancy resulted not from her ex-boyfriend Matt but from a man whose identity she has kept hidden.
That man turns out to be the school's new cross-country coach, Brandon Michaelson, who is in his twenties. Monica had a sexual relationship with him over the summer at her job at the New Haven Country Club. He texted her before school started to warn her about his new position, but they have not spoken since. Monica also learns he has a girlfriend. A quiet classmate named Ginny Cordero witnesses Monica leaving Brandon's office but assures her it is none of her business.
Monica has never accepted the official story that Jen killed herself and has spent years wondering why Jen left no suicide note. While searching Tom's desk, she discovers four anonymous letters, each mailed on the anniversary of Jen's death. Every envelope contains the same photo of the five cheerleaders and the typed message: "I KNOW IT WASN'T HIM. CONNECT THE DOTS" (26). In a locked drawer, she finds Jen's old cell phone. The texts have been deleted, but the call log shows an unrecognized number called Jen the morning she died.
Monica texts the number. The person claims to have been friends with Jen, refuses to identify themselves, and warns Monica not to trust Tom. Online research reveals that news articles describe Jack Canning as reaching into his dresser drawer when Tom shot him, not pointing a gun as Monica had always been told. Journalist Daphne Furman, who wrote a story questioning the investigation, tells Monica there was no forced entry at the Berrys' house and no bloody clothing found at Canning's home.
Through a note left in an unfinished house and a poem hidden in Jen's copy of
Wuthering Heights, Monica traces the anonymous texter to Ethan McCready, a former classmate of Jen's who was expelled after a girl reported him for writing cheerleaders' names in a notebook that the school treated as a hit list. Jen's name was the only one absent from the list.
As Monica investigates, she grows closer to Ginny, who remembers Jen's kindness from their shared gymnastics studio as children. Ginny becomes Monica's partner in uncovering the truth. Monica meets Ethan in person. He claims he saw Juliana arguing with a tall, hooded figure on the Berrys' back deck the night of the murders. He says he reported this to the police, but Tom intimidated him into silence. When Monica discovers the Berrys had a tall privacy fence blocking Ethan's supposed view from the woods, she confronts him. Ethan admits the truth: He was leaving Jen's bedroom that night. Jen had secretly invited him in while she was sick, and he climbed out her window onto the garage roof, from which he had a clear sightline over the fence. He and Jen had a secret bond; the night before homecoming, he visited her and they listened to an audiobook together.
Together, Monica and Ginny track down former cheerleader Carly Amato, now a nursing student. Carly initially claims she barely knew Juliana, but this proves false. Through Carly and another former squad member, Patrice Johnson, they learn that Carly introduced Juliana to two older college students, one of whom was dealing pills. Juliana witnessed a drug deal and wanted out. Carly refuses to name the men but confirms they drove a pickup truck.
Using a risky scheme, Monica and Ginny access the police database and print witness statements. An elderly neighbor's statement describes a pickup truck near the Berrys' house around 9:45 PM and a petite, dark-haired girl getting out and walking up the driveway. Since Juliana's father had already dropped the girls off earlier, the sighting suggests Juliana left the house and returned in someone else's vehicle, or that a third girl came that night.
At the homecoming parade, Monica spots Allie Lewandowski, the cheerleaders' former coach, with Brandon's arm around her waist. She realizes that Brandon is Allie's boyfriend, linking him to the circle of people connected to Juliana and Carly. After the homecoming dance, Brandon confronts Monica at a party, but she pulls away and calls her mother for a ride. At home, Monica retrieves a photo she took of Brandon over the summer and notices a crescent-shaped scar above his ankle, consistent with a dog bite. She connects the scar to the Berrys' Saint Bernard, found at the crime scene uninjured. A large dog would have attacked a stranger, suggesting the killer was someone the dog recognized.
Late one night, Tom catches Monica in his office. He admits he kept Jen's phone to review her call log, hoping to understand why she died. He acknowledges he has considered the possibility that Canning was innocent but says he believes the killer is dead. When Monica raises the question of the uninjured dog, Tom's expression makes clear he never fully considered its significance.
The next day, Brandon arrives while Monica's parents are out. She activates her phone's voice recorder and confronts him. Brandon confesses: He had a sexual relationship with 15-year-old Juliana, whom he met through Carly. When he tried to end it, Juliana threatened to tell Allie. The night of homecoming, he went to Susan's house to reason with Juliana. She became hysterical; he followed her inside and strangled her. Susan came downstairs and he killed her too. When Brandon attacks Monica, she fights back and screams for Petey to run for help. Ginny arrives and strikes Brandon with a baseball bat, knocking him unconscious.
Brandon is charged with assault and statutory rape, with murder charges expected. Tom is asked to step aside while the department reinvestigates the Canning shooting. Monica's mother reveals what she has long withheld: Jen did leave a suicide note, mentioning Monica and asking her to see California for her. Monica is devastated but finally confronts the certainty that Jen chose to die, not because someone killed her, but because grief and survivor's guilt were unbearable. In a final exchange, Ethan tells Monica that Jen called him the morning of her death to ask how the audiobook ended. He told her he would tell her when he saw her next, but he never did.
The novel's final chapter shifts to Ginny's perspective, five years earlier. Her father Phil Cordero, an abusive man with an alcohol addiction, picks her up while intoxicated and drives erratically in a rainstorm. He swerves into oncoming traffic, causing the collision that kills Bethany and Colleen. His truck flips over a guardrail toward a lake. Trapped upside down, he begs Ginny to unbuckle his seatbelt. She thinks of her mother's bruised face, turns, and walks away. The truck slides into the water. Ginny tells her mother her father beat her and left, and decides to bury the truth forever.