The Sisterhood of the Traveling Pants is a young adult novel that follows four friends Lena, Tibby, Bridget, and Carmen during the summer after their sophomore year of high school. To stay in touch while they spend the summer apart, they share a pair of pants which miraculously fits all of them despite their different body types. Each girl has her own adventures and grows up a lot over the course of the summer, but the power of the pants and their love for each other ensures that their friendship remains strong.
In the first novel of the series, the reader is introduced to four high school students: Lena Kaligaris, Tibby Rollins, Bridget Vreeland, and Carmen Lowell. They have been best friends since birth (their mothers attended prenatal exercise classes together). The summer before their junior year of high school, Carmen finds a pair of old jeans that fits each girl perfectly, despite their different sizes. This leads them to believe that the pants are magical. They share the "traveling pants" among themselves over the summer while they are separated.
Lena spends the summer with her grandparents in Santorini. During her stay, her grandmother attempts to set her up with a man by the name of Kostos. Kostos takes interest in Lena, who eventually returns the notion. She goes skinny-dipping and Kostos accidentally sees her. Later in the summer, Lena explains to her grandparents what happened in order to repair the rift between her and Kostos' grandparents, and confesses to Kostos that she loves him.
Tibby spends the summer working at a Wallman's store, planning to film a documentary of her experiences. She meets a 12-year-old girl, Bailey, after the latter faints at the store. (It is revealed that she has been diagnosed with leukemia.) Over the course of the summer, the two become close friends and Bailey begins to help Tibby film her documentary. Tragically, Bailey passes from her leukemia, which leads Tibby to refocus her documentary to capture the memories that they created together.
Carmen goes to South Carolina to spend the summer with her father, from whom she has grown apart since he and Carmen's mother divorced several years before. Carmen learns that he is engaged. Out of frustration at feeling left out of her father's new family, she breaks a window in their home with a rock and returns home to her mother. She eventually attends her father's wedding and reconciles with her father and his new family.
Bridget attends a soccer camp in Baja California. While there, she falls for one of the coaches, Eric Richman. Bridget pursues him in spite of the camp's prohibition on coaches and campers entering relationships with each other, and eventually sees him in his underwear. She conspires to lose her virginity to him, until Eric eventually tells her that he does not feel as if he can worship her as she deserves. Lena comes to comfort a depressed Bridget and ends up taking her home.