The following version of this book was used to create this study guide: McManus, Karen M. The Cousins. Delacorte Press, 2020. Kindle Edition.

As The Cousins begins, it appears as if Mildred Story has invited her teenage grandchildren, Milly, Aubrey, and Jonah, to spend the summer living and working at an island resort she owns. The invitation surprises the cousins because Mildred disinherited their parents years ago. The cousins have never met her. They met each other when they were children, but they have not seen each other since.

The teens do not want to go. However, their parents encourage them to accept the invitation. The cousins think that their parents hope to be re-inherited. Milly‘s mother bribes her by offering her a family heirloom. The only reason Aubrey is now anxious to go is because she has just learned that her father has had an affair with her swim coach, who is now pregnant with his child. Jonah complains about going because he has gotten a scholarship to attend a competitive science camp.

The three cousins communicate with each other through texts prior to their departure. Milly and Aubrey do not know what Jonah has grown up to look like because he does not have his picture on social media. The girls plan to take the ferry to the island together, but Jonah says he does not see the point of going with them. However, they bump into him on the ferry.

Jonah is better looking than the girls expected. They express their surprise, but accept that he is their cousin. However, they were right to be suspicious because the Jonah on the ferry is an imposter. His name is Jonah North. Their cousin Jonah bribed him to take his place so that he could attend the science camp.

The trio arrives on the island and encounters the old family doctor, Dr. Baxter. When he sees Milly, he thinks that she is her mother, Allison. He says she should not have come. He wants to know if Aubrey’s father, Adam, is there, too. Dr. Baxter’s granddaughter, Hazel, comes to get him and explains that he now suffers from early dementia.

Next, the cousins go to the resort where they are supposed to work. Up until now, they have been dealing with a man named Edward. Upon arriving, they meet with a man named Carson. Edward is no longer there, and Carson did not know they were coming. He is nice to them and tells them that their grandmother is on the premises. He brings Mildred to them, but it appears that their presence is also a surprise to her. Milly believes that Mildred did not send the invitation.

As the summer progresses, other strange things happen. Their grandmother leaves the island. Her lawyer, Donald, tries to get the cousins to leave by offering them another job. They stay. Milly discovers that Jonah is not their cousin. Milly and Aubrey keep his secret. They learn that Matt, Mildred's assistant's son, drowned on the same beach that Adam wrote about in a book he published. Additionally, a girl named Kayla, who knew their parents, died in a car accident around the same time that Mildred disinherited their parents. It seems as if Dr. Baxter knows a secret. The trio wonders if there is a connection between the deaths and Mildred’s action toward her children.

In the midst of all of this, Milly solves the mystery of who invited them to the island. It was their Uncle Archer who has been living and working on the island disguised as a bartender named Chaz. He has not seen Mildred. However, he thinks that Dr. Baxter recognized him. He is right. Dr. Baxter sends him a letter saying that he needs to tell him something that he should have told him years ago. However, Dr. Baxter dies before he can do so.

Mildred invites the teenagers into her home for brunch and to attend the Summer Gala. Things do not go well at either event. Mildred focuses her attention on Aubrey and Jonah. She ignores Milly. This upsets Milly and creates friction between her and Aubrey. Mildred and Donald catch Milly and Jonah kissing at the Summer Gala. At this point, they believe that Jonah is Milly’s cousin. Jonah tells Mildred that his father, Anders, hates her. Then Anders arrives with the real Jonah Story and tells everyone that Jonah North is an imposter. It turns out that Jonah North wanted revenge because Anders lost his family’s money through bad investments. Donald has everyone escorted away.

They are all supposed to leave the island. Instead, they stay with Archer. Mildred invites Aubrey to the house for one last visit. Aubrey realizes that Mildred is an imposter. She goes back the next morning to prove it. She is gone by the time the others get up, but Milly pieces together the truth and realizes that Aubrey is in danger. Archer goes to save her. Readers learn that Theresa has been posing as Mildred. Mildred Story died years ago. Donald and Mildred decided to disinherit the children. Theresa sought revenge because Matt drowned after Anders lured him into the water during a party at Cutty Beach. Kayla dated both Matt and Anders. Anders told her about his actions. Adam was with him at the time. Allison had gotten help. Anders confessed to her later, but she kept his secret. Donald killed Kayla and Dr. Baxter. Theresa’s sister, Paula, has been posing as Theresa.

While Theresa is telling Archer and Aubrey her story, Paula is setting fire to the house. Aubrey and Archer escape. Theresa dies. Paula escapes from the house and the police. The police arrest Donald. The house is uninsured; thus, the Story family's fortune is gone. The revelation of the truth helps Archer and Allison heal. Milly’s and Allison’s relationship also heals. Aubrey and Milly stay close; Milly and Jonah continue their romantic relationship. Paula sends a postcard to Jonah indicating that she plans to make Anders pay for his actions.