A few hours after nine-year-old Garnet Linden finds a silver thimble in the dried-up riverbed, the rains come and end the long drought on the farm. The rains bring safety for the crops and the livestock, and money for Garnet's father. Garnet can't help feeling that the thimble is a magic talisman, for the summer proves to be interesting and exciting in so many different ways.

There is the arrival of Eric, an orphan who becomes a member of the Linden family; the building of a new barn; and the county fair at which Garnet's carefully tended pig, Timmy, wins a blue ribbon. Every day brings adventure of some kind to Garnet and her best friend, Citronella. As far as Garnet is concerned, the thimble is responsible for each good thing that happens during this magic summer―her thimble summer.