The book follows the adventures of two young girls, Betsy and Tacy, who meet on Betsy's fifth birthday and become inseparable friends. Set in the early 1900s in the small town of Deep Valley, Minnesota, the story captures the whimsical and heartfelt moments of childhood as the pair navigates the joys and sorrows of growing up. Through their imaginative play and exploration, Betsy and Tacy's friendship blossoms, illustrating the enduring power of friendship and the beauty of a simple, happy childhood.