The following version of the novel was used to create this study guide: McFadden, Frieda. The Coworker. Hollywood Upstairs Press, August 29, 2023. Kindle.

In the psychological thriller The Coworker, author Frieda McFadden presents a "he said, she said" situation as Dawn Schiff and Natalie Farrell, both employees at Vixed, tell different stories about their relationship and their interactions with each other. Dawn makes it appear Natalie bullied her. Natalie argues she liked Dawn and tried to protect her from bullies. When Dawn disappears and Natalie is the prime suspect, it surfaces that both women are hiding secrets.

Since Dawn is generally punctual, Natalie is concerned when she is not at work on time. Additionally, Natalie answers Dawn’s phone and hears a voice she believes is Dawn’s begging for help. While Natalie is out of the office on sales calls, she stops at Dawn’s house to check on her. Dawn’s car is there, but no one answers the door. Natalie discovers the back door is unlocked and goes inside. She sees an uncorked bottle of wine on the kitchen counter. There are also two wine glasses, one broken on the floor. Natalie believes she knows Dawn well enough to know that she would not have left her kitchen like that. In the living room, Natalie finds blood on the carpet. She calls the police.

Natalie’s narration is interspersed with emails from Dawn to her best friend, Amelia “Mia” Hodge. Dawn describes how the employees at Vixed, particularly Natalie, are being increasingly cruel to Dawn. Dawn also tells Mia that she has discovered that Natalie is stealing from the company. Dawn has arranged a meeting with Seth to tell him about the theft. She also has asked Natalie to meet with her. Dawn plans to tell Natalie that she has discovered there is money missing. She wants to give Natalie a chance to confess to the theft and return the money before she tells Seth what is happening.

When detectives investigate Dawn’s house, they find Natalie’s fingerprints on several items there including one of the wine glasses and two knives. While Natalie did grab a knife from the block to protect herself in case an intruder was still there, she does not remember having touched another knife or either of the glasses. As Santoro, the detective in charge of the case, continues to investigate, he discovers Dawn’s emails describing the way Natalie bullied her at work. He learns that Dawn discovered Natalie was stealing from Vixed and that she intended to confront Dawn about it. He further finds Dawn’s blood and hair in the trunk of Natalie’s car. A body has been found in Cohasset that has been so badly beaten that the face is not identifiable and many of the teeth have been knocked out. Until authorities are able to positively identify the body, they assume that it is Dawn’s.

Natalie has been organizing an annual 5K charity to run to benefit cerebral palsy. She advertises that the cause is dear to her because her best friend in school, Amelia, suffered with cerebral palsy. Just before the race begins, with runners and spectators gathered around and television cameras recording, Santoro arrests Natalie for Dawn’s murder.

The novel takes a twist in Part 2 when it is revealed that Dawn is still alive. She had help from Mia’s brother, Caleb, in faking her death and framing Natalie for the murder. The two have framed Natalie because they want revenge for Mia’s death back in high school. Mia is the same girl Natalie claims was her best friend in school. In reality, Natalie and her real best friend, Tara Wilkes, bullied Mia to the point that Mia took her own life. Natalie was never punished for her role in Mia’s death, a lack of justice that Dawn and Caleb hope to correct by sending Natalie to jail for Dawn’s death.

Dawn knows that when it is discovered that the body found in Cohasset is not Dawn’s it will be much more difficult for Natalie to be charged with murder. Dawn has allowed herself to become too invested in punishing Natalie and decides that she must die if Natalie is to be punished properly. Caleb, who is in love with Dawn, goes to Natalie and Seth to help him find Dawn when he discovers Dawn is missing.

Natalie finds Dawn on a pier preparing to jump into the ocean. Natalie warns her that her death will be meaningless because Caleb told Natalie and Seth about their plan to frame Natalie for Dawn’s murder. Dawn decides she will kill Natalie with the concrete block she had tied to her ankle for her suicide attempt. Part of Caleb’s role in the plan to frame Natalie was to pretend to date Natalie so that he would have access to her house and objects with her fingerprints on them. Dawn has decided incorrectly that Caleb really loves Natalie and the best way to punish Natalie at this point is to kill her. Natalie stops Dawn’s murder attempt when she tells her that she does not want Caleb. She adds that Caleb has been crying because he is so concerned about Dawn.

When Natalie and Dawn reconnect with Caleb and Seth, Seth tells them that the body that was found actually belonged to Tara. Natalie recognizes the name and realizes from the look on Dawn’s face that Dawn killed Tara to punish her for the way she bullied Mia. It is discovered that Natalie is not as innocent as she appears. She admits through her narration that she did steal money from Vixed, but she returned it when she realized Dawn suspected her of theft. Further, Natalie has been stealing from the money she raised through the charity run. She indicates she believes Amelia owes her that for all the trouble she caused Natalie after she died by suicide.