The Summer I Turned Pretty follows protagonist Belly as she, her brother, and her mother join the Fisher family for a tradition in which they spend summers at Cousins Beach. Through a narrative interspersed with flashbacks of past summers, the novel focuses on Belly's coming-of-age, her close relationships with the Fisher brothers Conrad and Jeremiah, and a new romantic relationship with a new boy, Cam, that she meets.
At fifteen, going on sixteen years old, Belly arrives at the Fisher home during the fateful "summer she turns pretty." This summer will be different, she seems to intuit: Conrad is more distant and cold than usual, Susannah Fisher and Laurel have been spending less quality time with the kids, her brother has left early to go on his college tours, and she experiences a series of firsts that constitute a coming of age.
Whereas her past interactions with the Fisher brothers have always been buffered by her brother Steven, Belly now finds herself unsure of how to navigate her longstanding crush on Conrad. When Belly meets Cam, at a party, she finds Conrad reciprocating her feelings for the first time, out of jealousy. Over the course of the summer, she will navigate the steady affections of Cam and Jeremiah as well as the moody warmth of Conrad, which is the love she actually wants.
Meanwhile, at the end of the summer, she discovers that Susannah Fisher's cancer has returned, and that the doctors have given her a poor prognosis. Belly finally understands that Conrad's moodiness may perhaps have been his form of expressing pain at the thought of losing his mother.
By the time Belly finds out about Susannah's cancer, her romance with Cam has faded. Jeremiah, all of a sudden, confesses his romantic feelings for her. She disappoints him by wishing to remain friends. Before the summer ends, she resolves to confess her feelings to Conrad. He doesn't reciprocate these feelings the first time, but on a second try, in which Belly makes the move to kiss him, he begins to kiss her back. The summer ends with Belly and the two Fisher boys playing and singing in the pool.
Back home for the fall and winter seasons, Belly is surprised to receive a call from Conrad. He has driven all night to see her. She steps into his car and the novel ends with the expectant air of a new beginning.
