Adelheid (final letter pronounced as a t sound), alias Heidi, is a girl first raised by her aunt Dete in Maienfeld, Switzerland. Dete brings 5-year-old Heidi to her grandfather, who has been at odds with the villagers for years and lives in seclusion on the alm. This has earned him the nickname Alp-Öhi (Alp-uncle in the Graubünden dialect). He at first resents Heidis arrival, but the girl manages to penetrate his harsh exterior and subsequently has a delightful stay with him and her best friend, young Peter the goat-herd.

Dete returns three years later to bring Heidi to Frankfurt as a companion of a 12-year-old disabled girl named Clara Sesemann. Heidi spends a year with Clara, having conflict with the Sesemanns strict housekeeper Fraulein Rottenmeier and becoming more and more homesick. Her one diversion is learning to read and write, motivated by her desire to go home and read to Peters blind grandmother. Heidis increasingly failing health and several instances of sleepwalking prompt Claras doctor to send her home to her grandfather. Her return prompts the grandfather to descend to the village for the first time in years, marking an end to his seclusion.

Heidi and Clara continue to contact each other. A visit by the doctor to Heidi and her grandfather convinces him to recommend Clara to visit Heidi. Meanwhile, Heidi teaches Peter to read and write. Clara makes the journey the next season and spends a wonderful summer with Heidi. Clara becomes stronger on goats milk and fresh mountain air, but Peter, feeling deprived of Heidis attention, pushes Claras wheelchair down the mountain to its destruction. Without her wheelchair, Clara attempts to walk and is gradually successful. Claras grandmother and father are amazed and overcome with joy to see Clara walking. Claras wealthy family promises to provide a shelter for Heidi, in case her grandfather will no longer be able to do so.