The following version of this book was used to create this study guide: Glasgow, Kathleen; Lawson, Liz. The Agathas New York: Delacorte Press, 2022.

The novel is narrated in first-person present by the two main characters, Alice and Iris. The girls take turns narrating every other chapter throughout the novel. The book starts on the morning of Halloween. Alice Ogilvie returns to Castle Cove High after spending the beginning of her junior year on house arrest. After learning that her ex-boyfriend, Steve, had started dating her best friend, Brooke, while she was in Egypt with her emotionally distant mother, Alice decided to get revenge on them by faking her own disappearance, just like her favorite author, Agatha Christie, once did. She broke a vase in her house and crashed her car on the side of the road, then disappeared for five days. The town panicked and a search party was sent out for her. When she returned and admitted she had faked the whole thing, she was put on house arrest and all her old friends decided she was crazy. She spent the whole of house arrest reading Agatha Christie books and ignoring the new horse her parents bought her so she would not be bored. When she returns, her guidance counselor, Ms. Westmacott informs her that she is going to be assigned a tutor: Iris Adams.

Iris takes the job tutoring Alice so she can save up enough money to escape Castle Cove with her mother and get away from her abusive father. Although there is a restraining order preventing him from contacting the family, he hovers around and threatens to return, which frightens Iris.

On Halloween night, Alice decides to confront her friends at their big part at Levy Castle. However, when she arrives, she gets into an argument with Brooke and Steve. She leaves and goes home at the same time Iris walks down the highway from her apartment because she had a panic attack about her father and needed some air. When Iris arrives in front of Levy Castle, she is shocked to see Steve and Brooke in a physical fight. Brooke breaks away from Steve and runs into the woods.

The next day at school, Brooke is nowhere to be found. Because of Alice’s recent fake disappearance, no one takes Brooke’s disappearance seriously. Alice calls Brooke’s grandmother, Lilian Levy, who comes to town and offers a $50,000 reward for information on Brooke. Iris wants the money to escape town, and Alice wants to play detective, plus both of them think the police are incompetent, so the two girls set out to find her on their own.

The girls invite Iris’s friends from the forensics club, the Zoners, to help them look for Brooke and they find her body in the ocean. Steve is arrested for her murder, but neither Iris nor Alice believes he is responsible for the crime. The girls go to jail to speak with Steve, who tells them that he only sipped a single beer on Halloween but then blacked out and woke up in the woods. The girls leave and meet with Steve’s lawyer, Ricky Randall, who used to be Iris’s babysitter. She tells them that the police found one of Steve’s shoes at the Overlook where Brooke fell from.

The girls get burner phones and text Alice’s former friends to see if they had anything to do with Brooke’s disappearance. Park admits that she had tried to drug Brooke so she could hook up with Steve, but Steve drank the Ambien-spiked beer instead. Kennedy tells Alice that she left the Halloween party so she could make a sex tape with Cole Fielding, and she shows Alice the time-stamped video as evidence.

The girls decide to focus their attention on Brooke’s father, Coach Donovan. Alice goes inside his house to talk about Brooke’s funeral and spikes his drink with Xanax. When he falls asleep, she lets Iris in and they go through his office and steal his phone. The girls go to Alice’s house to meet with their friends and Ricky Randall, who agrees to trade information with them. They learn that Brooke texted her father for a ride on Halloween. They also learn Coach Donovan was in debt for hundreds of thousands of dollars. His wife had left all her money to Brooke. He got a stipend each month, but that would end after Brooke turned 18, unless she died before then—in which case he would keep everything.

Steve tells Ricky that he is just going to plead guilty. The girls learn that Coach Donovan has a secret identity as the son of the richest woman in the country, and that woman visited Steve to bribe him to plead guilty. Ricky tells Alice and Iris to take everything they have learned to the local news so they can put pressure on the police to look at Coach Donovan rather that Steve. The police arrest Coach Donovan and let Steve go.

Thinking they have succeeded, Iris prepares to leave with her mother to Whispering Pines up north. Alice learns about her plan and the two girls get into a horrible fight.

Alice spends Thanksgiving alone because her parents are out of town. She gets depressed and decides to go see Iris. She walks into a fight between Iris and her father and calls the police. Iris’s father is arrested, but Iris still refuses to talk to Alice.

Hurt, Alice goes to the horse stable to spend some time with her horse. However, she hears Ms. Westmacott admitting to killing Brooke to her horse. Ms. Westmacott realizes Alice heard her and tells her that she was trying to talk to Brooke about dating Coach Donovan, but Brooke was rude to her, so she killed her and hoped it would enable her and Coach Donovan to be together. Ms. Westmacott attacks Alice and tries to run her over with her car, but Alice jumps on her horse and chases her guidance counselor, who loves horses much more than people and would do anything to not hurt a horse. Ms. Westmacott crashes her car into a tree and Alice ties her up.

In the end, Ms. Westmacott is arrested and Coach Donovan is released. Alice and Iris make up, and Iris collects the reward money. The girls plan to solve more mysteries together.