The following version of the novel was used to create this study guide: Kubica, Mary. The Other Mrs.. Park Row, February 18, 2020. Kindle.

In the psychological thriller The Other Mrs. by Mary Kubica, Sadie Foust becomes obsessed with the murder of a woman who lived down the street from her family’s new home in Maine. Sadie fears the murderer is still lurking about the neighborhood, perhaps even living in her home. The truth, however, is closer than even Sadie realizes.

Sadie and her family moved to Maine hoping for a fresh start. Sadie’s husband, Will, had an affair. Her oldest son, Tate, was expelled for taking a knife to school. Sadie, a doctor, had a nervous breakdown and allowed a patient to die when she left him in the care of a resident.

Just a few weeks after the move, Morgan Baines, a neighbor, is violently murdered. Sadie is already uncomfortable in Maine. She and Will inherited the house that belonged to Will’s sister, Alice. Alice died by suicide leaving her daughter, Imogen, in Will's and Sadie’s care. Imogen is an angry teen girl who does her best to make Sadie feel unwelcome.

Sadie becomes obsessed with Morgan’s murder, especially after a next-door neighbor claims he saw Sadie fighting with Morgan. Sadie claims she was at work the day, but the receptionist tells police that Sadie was out of the office for a three-hour block of time during the day. Sadie has no knowledge of having left the house.

Kubica builds her novel using the first-person point of view of four different characters. Sadie is the central narrator. She suffers with dissociative identity disorder but is not aware of her condition. Camille is one of Sadie’s alter personalities, a provocative, loud woman who admits she is obsessed with Will. Mouse, another alter, is a young girl who is being terribly abused by her new stepmother. Will, Sadie’s husband, knows Sadie has a mental disorder that causes her to behave like different people though he did not have a name for that disorder. He uses Sadie's disorder to manipulate her into committing crimes for him.

Will is the antagonist in the novel as he killed Erin, his first fiancee after he learned she had fallen in love with another man and wanted to break off the engagement. Erin’s younger sister suspected Will killed Erin, but was unable to prove it.

Will recognized that Camille was willing to kill for him when Carrie Laemmer, a student of his, accused him of sexual harassment. Will describe to Camille what it would do to him and his reputation if he was found guilty of the charges. Camille arranged for Carrie to drown in a canal near campus.

In the present, in Maine, Will learned too late that Erin’s younger sister, Morgan, lived down the street from his family’s new house. Morgan began threatening to tell Sadie what Will had done to Erin. To keep that from happening, Will told Camille what was happening to him, and how Morgan wanted to ruin his life. Just as Camille killed Carrie, Camille killed Morgan.

As the police focus on Sadie as Morgan’s killer after the neighbor claimed he heard the two women fighting, Sadie decides that she has to fight to protect her own name. She goes to the police after she finds a bloody washcloth in the laundry and her dogs dig up a knife fitting the description of the one that killed Morgan in the backyard.

As Sadie talks to Officer Berg, she learns that Will is undermining her, trying to make her appear guilty. As Sadie is talking to Berg, she transforms into her alter of Camille, prompting Berg to call for a psychiatrist. She works with Sadie and learns she has three distinct personalities. She tells Sadie that she suffers with dissociative identity disorder. The psychiatrist believes it is Camille who is responsible for Morgan’s murder.

Because the police have no concrete evidence linking Sadie to Morgan’s murder, they cannot hold her when she requests to go home. At home, she begins questioning Will and putting together the pieces of what really happened to Morgan. She learns that Will was aware of her disorder, but never told her or tried to get help for her. Because Sadie is so close to realizing that Will instigated Camille to murder Morgan so he could continue to cover his murder of Erin, Will decides that he needs to kill Sadie as well.

As Sadie fights for her life and the futures of her children against Will, Will tells her how he killed Erin and how Camille killed Morgan. Will is unaware that Imogen is recording his confession from the next room. Imogen saves Sadie’s life by hitting Will in the head with a fireplace poker and incapacitating him. Sadie grabs a knife from the kitchen cabinet and kills Will. She tells him that he deserves to die.