It follows the adventures of a twelve-year-old American girl, Katy Carr, and her family who live in the fictional lakeside Ohio town of Burnet in the 1860s.

When a terrible accident makes her an invalid, her illness and four-year recovery gradually teach her to be as good and kind as she has always wanted.

12-year-old Katy Carr lives with her widowed father and her two brothers and three sisters in Burnet, a small Midwestern American town.

She behaves somewhat kindly to the children and dreams of some day doing something "grand" with her life: painting famous pictures, saving the lives of drowning people, or leading a crusade on a white horse.

The very next day, however, Katy wakes in an ill humor, quarrels with her aunt and pushes her little sister so hard that she falls down half a dozen steps.

Had Aunt Izzie actually explained that the swing was unsafe because one of the staples supporting it had cracked, "all would have been right," but she believes that children should unquestioningly obey their elders.

Helen tells Katy that she is now a student in the "School of Pain" where she will learn lessons in patience, cheerfulness, hopefulness, neatness, and making the best of things.

With Cousin Helen's help, Katy makes her room tidy and nice to visit and gradually all the children gravitate to it, coming in to see her whenever they can.

At the beginning of the book she is a twelve-year-old tomboy, who much prefers running around outdoors to quiet 'ladylike' pursuits, and so tears her clothes and is always untidy; however, she longs to be good.

Elsie Carr: the third sister, an awkward child at the beginning of the book, too old to play with the 'babies' and too young to be included in Katy and Clover's games.

Papa (Dr Philip Carr): the children's father, a very busy doctor who has been a single parent, firm but understanding, since his wife's death when Katy was eight.

Susan Coolidge shared her publisher, Roberts Brothers, with Louisa May Alcott, and What Katy Did helped satisfy the demand for naturalistic novels about girlhood that followed the 1868 success of Little Women.

Katy's hair was forever in a snarl; her gowns were always catching on nails and 'tearing themselves'; and, in spite of her age and size, she was as heedless and innocent as a child of six.

Happily, her head was so full of other things, of plans and schemes and fancies of all sorts, that she didn't often take time to remember how tall she was.

The most recent film (1999) starred Alison Pill as Katy, with Kevin Whately as Papa, Megan Follows as Cousin Helen, Michael Cera as Dorry, Bryn McAuley as Joanna, and Dean Stockwell as "Tramp".