The Penderwicks -- responsible older sister Rosalind, temperamental Skye, budding writer Jane, little Batty, and their botanist father (their mother is dead) -- head out for a summer vacation at a rented cottage on the Arundel estate in the Berkshires. There they meet Jeffrey, the musically talented son of Mrs. Tifton, the snobbish owner of the estate, and Cagney, the teen groundskeeper. After some initial social disasters, the girls and Jeffrey get to be good friends, and Rosalind develops a crush on Cagney.
But Mrs. Tifton, supported by her equally mean and stuck-up beau Dexter Dupree, doesn't approve of the Penderwick girls or their friendship with her son. After a miserable and disastrous birthday party for Jeffrey, they overhear his mom and Dupree planning to marry and send Jeffrey away to military school. Gentle, artistic Jeffrey is horrified, and when his mother won't listen to his pleas, he plans to run away.