In the third summer of the Sisterhood of the Traveling pants, the Septembers graduate high school and spend their summer preparing to go away to university in September. Bridget is off to soccer camp as a coach, while Lena, Tibby and Carmen spend the summer at home working their summers away. The group has a plan to meet up the last weekend before they all leave at a cabin, as their last getaway together before things change completely for them.
After the death of her Bapi the summer before, Lena’s dad forced his mother, Valia, to come and live with them even though it was clear she didn’t want to. Thanks to that decision, Lena got a summer job at a Italian restaurant that she hates to stay out of the house and away from the furious Valia. In preparation for her schooling, Lena takes a art class that her father doesn’t approve of. Lena’s dad also decides that he won’t pay for her to go to school, which forces Lena to create a portfolio to be considered for a scholarship to RISD.
Bridget heads to a soccer camp in Pennsylvania where she she will be a camp counselor, and is head of a soccer team for the duration of the summer. In true Bee fashion, she read next to nothing that was sent out to her. If she had, she would have known that someone from her past would also be at the soccer camp: Eric. She has a bit of a crisis, as not only is she still in love with him. But, he has a girlfriend. They also get paired up to supervise events while not coaching, and slowly become friends. After a summer where Bridget controls herself, it seems like the work they did could all be unraveled when Bridget becomes sick.
Tibby and Brian’s relationship seems to be changing slowly after the pair go to the senoir party together, though it’s quickly overshadowed for Tibby when her sister, Katherine, falls out of her window in an attempt to pick an apple from the apple tree that is outside of Tibby’s window. Katherine ends up being fine, but Tibby can’t seem to let it go. She spends the summer punishing herself over Katherine’s fall, and avoids Brian as a continued means of torturing herself. Slowly, with the helps of her friends, Katherine and Brian, Tibby eventually forgives herself and gets ready for film school.
Carmen’s summer is spent taking care of Lena’s Grandma, Valia, and resenting her mother and her new husband, David, drop a bomb shell on her after graduation: Carmen’s mom is pregnant. After hearing the news, Carmen changes her plans of going away to Williams University and applies to University of Maryland instead. While she is taking care of Valia, Carmen meets a boy named Win who she instantly has a crush on. She spends the summer showing him ‘Good Carmen’, and feels like she is lying to him. Her mom gives birth in a very dramatic way, and Carmen instantly falls in love with her baby brother.
In true Septembers fashion, the girls spend their last weekend together and say goodbye to each other for now. They exchange gifts for the others to hold them close, and embark on the next chapter of their lives.