As far as teenager Isabel Conklin is concerned, summer is the only season that matters. For Isabel, nicknamed “Belly,” winter and spring are merely waiting periods. Not only is her birthday in August, but each June, she, her mother, Laurel, and her brother, Steven, travel to Cousins Beach to spend three months with the Fisher family, at their beach house on the Atlantic Coast.  

  Laurel Conklin and Susannah Fisher have been best friends since childhood, and now spend summers together with their children. Susannah has two teenage sons: Conrad, the eldest, is about to attend his freshman year of college, and Jeremiah is still in high school. The Fisher boys have been a source of friendship and fascination for Belly for as long as she can remember, and she’s been harboring a crush on Conrad for years. Both families have faced challenges; Susannah had cancer, and the Conklins recently divorced. 

The summer that Belly turns sixteen, major changes impact everyone staying at the Fisher’s beach house. After the Conklins arrive, the two families begin to settle in for what at first seems like a typical vacation—but it soon becomes clear that this summer won’t be the same as those before. Belly has blossomed into a beautiful young woman, and both boys notice the change in her appearance. However, Conrad’s moodiness makes Belly feel more distant from him than ever before. She learns that Conrad has quit the football team, broken up with his girlfriend, and started smoking. Initially, the cause of Conrad’s sullen mood is unclear. As the story progresses, the reader learns more about the difficulties facing the Fisher family. 

Fearing that Conrad will never reciprocate her feelings, Belly tries to ignore her crush on him. Soon after her brother Steven leaves to tour colleges with their father, Belly begins to pursue a relationship with a boy named Cam. They meet at a beach party and realize they had met once before when they were thirteen, at a convention for Latin students in Washington, D.C. Cam and Belly begin to date, and she likes that he is different from both Jeremiah and Conrad. Cam is “straight edge,” which means that he does not drink alcohol, do drugs, or practice other risky behavior. Although she appreciates this, Belly also becomes frustrated with the lack of physical intimacy between the two of them. She wishes that he would kiss her without asking first and take part in more risqué activities like skinny-dipping. 

Flashbacks throughout the novel show moments from different summers from the years that Belly was age ten through fourteen. Several chapters focus on the previous summer, when Belly’s best friend Taylor comes to Cousins Beach to visit for a week. In the past, Belly was something of a non-threatening sidekick for her pretty friend. However, during the visit, Taylor notices that Belly is becoming attractive in her own right and begins to see her as competition for male attention. On the other hand, Belly unfavorably compares her own appearance to that of her friend, who wears makeup and flattering clothing. Belly has her first kiss with Jeremiah during a game of truth or dare, and is mortified that Conrad and Steven watch it happen. By the end of Taylor’s visit, the two girls are still friends, but are not as close as they used to be. 

Although Belly and Cam date throughout the summer, romantic tension between Belly and Conrad grows. At a house party, Conrad becomes aggressive with a young man in his twenties. To prevent them from coming to blows, Belly steps between them to break up the argument. Later, as she sits alone with Conrad in a car, he gently strokes her hair. He is on the verge of saying something to her, but the moment passes. 

Toward the end of the novel, Belly’s attraction to Cam wanes, and after Jeremiah reveals his interest in her, she can no longer deny her crush on Conrad. Unwilling to let her affection remain unspoken, Belly confronts Conrad and tells him that she loves him. He rejects her, which leads to a physical altercation between the Fisher brothers. After Laurel breaks up the fight, Belly discovers the secret behind underlying tension in the house; Susannah’s cancer has returned, and she does not have long to live. Belly switches her attention from romance to supporting the Fisher family through their painful time. She comforts Jeremiah, promises Susannah that she’ll look after Conrad when she’s gone, and listens to Conrad express his devastation. Belly and Conrad share a kiss, but Conrad explains he is too distraught about his mother to start a relationship now. 

At the end of the summer, the Fishers and Conklins clean up the house and spend one last day at Cousins Beach. Several months later, Conrad drives from Boston to see Belly in the middle of winter. After her mother goes to sleep, Belly sneaks out of her house to meet him, suggesting the start of a relationship between the two teenagers.