Peck has expressed that "a young character's journey into maturity" remains the unwavering theme of the adolescent novel. He adds that the essence of his stories revolves around the idea that "you will never grow up until you begin to think and act independently of your peers." In Those Summer Girls I Never Met, the protagonist Drew Wingate presents a curious contradiction to this notion. Drew finds himself without peers, save for his younger sister Stephanie, who joins him on a cruise at the behest of their grandmother. Temporarily, both siblings are thrust into a world of adults.

In this narrative, the most pivotal figures belong to the golden age of their sixties and seventies. At the heart is their grandmother, Connie Carlson, a chanteuse from the Big Band era. Due to a rift between Connie and the children's mother, Stephanie and Drew have barely had a glimpse into their grandmother's life. Initially, they resist Connie's attempts to usher them into adulthood: she bestows upon Drew a white dinner jacket and Stephanie her first pair of heels. Connie's philosophy, "On a cruise, you're any age you can get away with," challenges them. Together with their spirited grandmother, Drew and Stephanie explore their dreams amid the reality of their responsibilities. Nevertheless, Peck sees no reason for despair in this situation: he believes that while freedom from leads to tedium, freedom to act holds endless fascination.

The novel unfolds with humor, largely thanks to Drew's comedic self-awareness and his knack for stepping back to relish the absurdity of his circumstances. Yet, it is tinged with sorrow. Connie's battle with cancer and Drew's grandfather Shep's struggle with alcoholism are somber issues that the characters endure and come to partially comprehend.