"Thimble Summer" is the story of Garnet Linden, a young girl living on a Wisconsin farm during the 1930s.  The narrative covers the events of Garnet's 10th summer, exploring themes of rural life, family, and growing up.

The story begins during a terrible drought; the weather is withering the crops, and Garnet's father is worried about how the family will get by.  Fortunately, the rains come, the crops are saved, and Garnet experiences a summer filled with exploration and discovery.

Garnet is an active child who loves the freedom and activity of farm life.  She enjoys reading, sharing adventures with her best friend Citronella, and is always trying to keep up with her older brother Jay.  The arrival of Eric, an orphan boy living on his own, at first causes difficulties in Garnet's relationship with her brother, but she soon learns to like Eric and accepts him as a member of the family.  Garnet has an independent spirit, and sometimes acts rashly without thinking.  When her brother angers her near the story's end, Garnet runs away to the city, and realizes the seriousness of her actions only when she returns. 

The growth Garnet experiences during her "thimble summer" sets her well on the road to adulthood.  She demonstrates her capability and maturity by showing her pig at the fair, and proudly treats her family to a party supplied by her winnings.