Tibby Tomko-Rollins, Lena Kaligaris, Carmen Lowell and Bridget Vreeland have been best friends since childhood. In the beginning of their summer vacations, they find a pair of jeans that fits each one of them perfectly; they decide to share the pants as a magic symbol of their friendship and form a sisterhood with a manifesto of ten rules. Bridget travels to a soccer camp in Mexico and she has a crush on the coach Eric, who is older than her. Lena visits her family in Greece and has a crush on the local fisherman Kostas and finds that there is a feud between their families. Carmen travels to South Caroline to spend the vacation with her father and she finds that he will marry the divorced Lydia Rodman. Tibby stays in town working in a department store to raise money to buy new equipment for her documentary and is befriended by the twelve year old Bailey that is very ill. After their vacations, they grow-up and their friendship remains solid as never.