The following version of this book was used to create the guide: Jackson, Holly. A Good Girl's Guide to Murder. Delacorte Press, 2019.

In Holly Jackson's A Good Girl's Guide to Murder, 18-year-old Pippa Fitz-Amobi uses her senior capstone project to investigate the truth of Andie Bell's disappearance and Sal Singh's death five years prior to the narrative present. Though Sal was convicted of Andie's murder just before the cops discovered his body in the woods, Pip never believed Sal was capable of murder. She began her research by asking Ravi Singh, Sal's brother for help, expressing her determination to prove Sal's innocence.

On her computer, Pip began a series of entry logs, recording the data she collected and information she uncovered. In the logs, she transcribed her interviews, and wrote out her theories regarding Andie and Sal's deaths. According to police reports, Sal originally had an alibi for the night Andie disappeared. He told the cops he was with his friends, Max, Naomi, Millie, and Jake. While the friends initially corroborated his alibi, the next day they told the police they had lied: Sal had left the house earlier than he originally said. Suspecting Sal because of his falsified alibi, and his romantic involvement with Andie, the cops interviewed him.

When Pip recovered the interview, she grew worried. Sal sounded guiltier than she had hoped. However, she decided not to lose hope, remembering her promise to Ravi. She carried on with her investigations, interviewing Andie's former best friends, as well as Max and Naomi. Through these sources, Pip learned that Andie's father had been abusive, and that Max and Naomi did not seem trustworthy. She added all of their names to her persons of interest list. Through Andie's friends she also discovered that Andie had been cheating on Sal with a mysterious older man. Initially suspecting her high school history teacher and best friends Cara and Naomi's dad, Elliot Ward, Pip cleared his name; Mr. Ward denied having known Andie. Max also denied knowing her, which Pip later falsified.

Through further investigations, Pip and Ravi learned that Andie had been selling drugs at high school house parties. Pip decided to attend a party with her friend to see if anyone was still selling. Through a classmate, she retrieved the number of local dealer, Howie, as well as his primary dealing location. Alone, Pip stalked Howie in an empty lot. She photographed Howie completing several transactions, then followed him on foot to his house. With Rav’s help, she used the photos as leverage to get information from Howie.

Their conversation with Howie, led Pip and Ravi to break into the Bell house in search of Andie's old burner phone. They did not find the phone, but they took pictures of Andie's planner. Through these photos, Pip discovers that Andie was meeting her mystery lover at a hotel in a neighboring town. Meanwhile, Pip began receiving an increasing number of anonymous notes in her sleeping bag, locker, and desk, warning her to stop digging into Andie and Sal's case. The notes were followed by an even more upsetting series of text messages from an unknown number. Pip hid these threats from her friends, determined to find the truth.

One night, after Pip's dog went missing on a walk, Pip received another text insisting she destroy her laptop and capstone files if she ever wanted to see Barney again. Terrified, Pip obeyed. However, Barney turned up dead in the river the next day. Her dog's death led Pip to swear off the case. Ravi realized the only reason Pip would quit was if she had been threatened. He convinced her to continue investigating with him in secret.

After receiving yet another warning note, Pip became more unnerved. Suspecting Naomi's involvement, Pip traced the print history on the Ward family computer, only to discover Mr. Ward was behind the notes. She realized he was the murderer. With Ravi's help, Pip tracked Mr. Ward's car, discovering that he was not in fact tutoring during his nights away from home, but was visiting his former family home in New Canaan. The next night Pip followed Mr. Ward to the house, called the police, and knocked on the door. Inside, she demanded Mr. Ward explain himself. He confessed to having been involved with Andie before her disappearance.

When Andie threatened to turn him in, the two fought; Andie fell, injured her head, and fled. When she failed to turn up in the days following, Mr. Ward became convinced he had killed her. He blackmailed Naomi and Max into framing Sal. Then he lured Sal into the woods and murdered him to cover his involvement with Andie. A few days later, he found a girl, who believed to be Andie, and locked her in the New Canaan home. The police arrived and arrested Mr. Ward. In the attic, Pip discovered the kidnapped girl was not Andie, but only looked like her.

The next day, Pip was haunted by the fact that Andie's murderer was still at large. Though Ravi urged her to let the police handle it, Pip could not let it go. Looking through her files again, she realized Becca was the real killer. She visited Becca, and got her to talk. Becca admitted that Andie had returned to the home after her argument with Mr. Ward. The sisters got into a fight, during which Andie fell, hit her head again, vomited, and choked to death. So angry with her sister, Becca let Andie die. She drove her body to an abandoned farmhouse, where she dumped her into the septic tank. Though Pip tried showing Becca grace, Becca had already drugged her. She chased Pip into the woods, where she nearly strangled her to death. The police arrived in time and arrested Becca.

Two months later, Pip presented her project to the school. She talked about the lessons she learned from her investigations. She and Ravi gave Sal the tribute he deserved.