All In

All In (2015) is a young adult psychological thriller by Jennifer Lynn Barnes and the third installment in the Naturals series. The story follows the Naturals, a group of gifted teenagers who consult for the FBI, as they are called to Las Vegas to hunt a serial killer whose methods are rooted in a complex mathematical pattern. At the same time, protagonist Cassie Hobbes must grapple with a shocking development in her mother’s cold case. The novel examines The Redefinition of Family Through Shared Trauma and Trust, The Relationship Between Talent and Trauma, and The Inevitable Collapse of Ordered Systems of Violence.

Barnes is a #1 New York Times bestselling author, also known for her highly popular series The Inheritance Games. Her academic background, which includes a PhD from Yale University and advanced degrees in psychology and cognitive science, provides a foundation for the novel’s exploration of criminal profiling.

This guide is based on the 2023 Little, Brown and Company revised trade paperback edition.

Content Warning: The source text and this guide contain depictions of graphic violence, physical abuse, emotional abuse, child abuse, cursing, and illness or death.

Plot Summary

The narrative is primarily from the perspective of Cassie Hobbes, a teenage profiler for the FBI’s Naturals program, with some chapters from the point of view of the unidentified serial killer the team is hunting.

Cassie is home for New Year’s with her paternal grandmother, Nonna, when her father, Vincent Battaglia, arrives unexpectedly from his overseas military post. He informs Cassie that a body believed to be her mother, Lorelai Hobbes, has been found five years after Lorelai’s violent disappearance and presumed death. He shows Cassie a necklace found with the remains, but she refuses to take it, insisting it is evidence. Distraught and seeking answers, Cassie decides to return to the Naturals program to investigate.

Judd Hawkins, the ex-marine who serves as the team’s guardian, picks Cassie up. On a private jet, she rejoins her fellow Naturals: profiler Dean Redding, lie detector Lia Zhang, and statistical genius Sloane Tavish. Their supervisors, Agents Veronica Sterling and Tanner Briggs, are also present. The fifth Natural, emotion reader Michael Townsend, is absent; he went home for Christmas, a fact that worries the team due to his abusive family life. On the plane, Agents Briggs and Sterling brief the team on a new case in Las Vegas, where a serial killer has murdered three people—Alexandra Ruiz, Sylvester Wilde, and Eugene Lockhart—in three days at three different casinos. Each victim was killed via a different method and marked with a four-digit number.

In Las Vegas, Michael rejoins the group, having used his father’s connections to secure a luxury suite at the Majesty casino. The team’s personal struggles soon come to the forefront. Sloane is anxious about being in her hometown; her estranged and powerful father, Grayson Shaw, owns the Majesty, while her half-brother, Aaron Shaw, works at the casino but is unaware of her existence. Meanwhile, Cassie overhears Lia discover fresh bruises on Michael’s torso, evidence of his father’s abuse. As Cassie grapples with her own grief, Agent Sterling gives her a USB drive containing the files on her mother’s case.

The team begins its investigation, identifying persons of interest that include amateur poker player Beau Donovan, actress Camille Holt, and stage magician Tory Howard. Sloane deciphers the numbers on the victims’ wrists as being derived from the Fibonacci sequence; she later determines that the murder locations form a Fibonacci spiral, allowing her to predict the next crime scene.

The killer strikes again, murdering Camille Holt by strangling her with her own necklace and carving the next number in the sequence into her wrist. The violence of the attack suggests the killer’s original plan was thwarted. As the investigation intensifies, the team’s personal traumas add to the tension: Sloane’s distress over her family leads her to shoplift; Lia confronts Cassie about her unprocessed grief, urging her to remain focused.

After several disappointing red herrings, a major breakthrough occurs when Sloane hacks into FBI and Interpol databases, discovering a dozen unsolved serial murder cases spanning 60 years. All the cases follow a distinct pattern: A new case of nine murders began every three years on Fibonacci-derived dates, with each case using one of seven prescribed methods that repeated in a 21-year cycle. The team realizes they are dealing with a secret society or cult of serial killers. They theorize that the Las Vegas killer is not a member but is escalating the cult’s ritualistic pattern to attract their attention. The cult’s structure appears to consist of killers known as the Seven Masters, a female oracle called the Pythia who bears the heir, and the heir, known as Nine.

The case becomes personal when Michael is marked with a poison-ivy-based rash forming a Fibonacci number, identifying him as the next intended victim. The FBI sets up a sting operation at the Majesty’s Grand Ballroom, the predicted location of the next murder, with Michael acting as bait. During the event, the power is cut. In the chaos, Cassie realizes the killer is Beau Donovan and that he is targeting Aaron Shaw. When the lights come back on, Aaron is dead.

Beau is arrested, but the evidence against him is slim without witnesses. The team deduces that he planted the bloody plastic wrapping from the murder weapon on a bystander. Tory Howard, who is Beau’s foster sister and was secretly in a relationship with Aaron, agrees to testify against Beau. During interrogation, Beau reveals a scar on his chest, a symbol of seven circles around a cross. Cassie recognizes the symbol from Beau’s chest as the same one carved on her mother’s coffin. Beau is suddenly poisoned in his cell, dying before he can be tried—an act the team attributes to the cult.

The notorious serial killer Nightshade, who murdered Judd’s daughter Scarlett years prior, reveals he is in Vegas and has been monitoring the team. He poisons Agent Briggs and demands to speak with Cassie in exchange for the antidote. During their confrontation, Nightshade confirms the cult’s structure and reveals that a new Pythia must kill her predecessor to take her place. Nightshade implies that Cassie’s mother is alive and became the new Pythia after being forced to kill the previous one, staging her own death in the process.

The FBI finds a woman, hired by Nightshade as a nanny, murdered in a hotel room. A young girl named Laurel is with her. Laurel, who reveals her cult name is “Nine,” shows Cassie a locket containing a recent photograph of herself with Cassie’s mother, confirming Lorelai is alive. Cassie understands that the body buried in her mother’s grave was the previous Pythia. To protect Laurel and deceive the cult, Cassie proceeds with a funeral for the unidentified remains. The novel concludes with Cassie and the Naturals resolved to find the Masters, rescue her mother, and protect Laurel from the cult.