Introduction to the Penderwick Sisters: The story follows the adventures of four sisters (Rosalind, Skye, Jane, and Batty) during their summer vacation at a beautiful estate called Arundel.

Friendship with Jeffrey: The sisters befriend Jeffrey, the son of the Arundel estate's owner (Mrs. Tifton) and together they embark on various escapades, exploring the estate and its surroundings.

Conflict and Resolution: The children's fun is threatened by Mrs. Tifton, Jeffrey's strict and snobbish mother, who disapproves of the Penderwicks' influence on her son. The sisters work together to help Jeffrey stand up to his mother and assert his desire to not attend military school.

Themes of Family and Friendship: Throughout the book, themes of family bonds, loyalty, and the value of true friendship are explored as the Penderwick sisters support each other and their new friend through various challenges.