Nils Holgersson is a lazy, disrespectful fourteen-year-old farm boy who torments animals on his family's farm, revealing his selfish and mischievous nature. This cruelty leads him to capture a tomte, a mythological creature. When Nils refuses the tomte's offer of a gold coin for freedom, the tomte shrinks him to miniature size and grants him the ability to talk with animals. Seeking revenge, the farm animals force the tiny Nils to flee by climbing onto Morten, a domestic white goose trying to join migrating wild geese.

Accompanied by his pet hamster Carrot, Nils journeys across Sweden with Morten and the wild geese flock led by Akka. Distrusted by the geese due to his human nature and cruelty, Nils must prove his worth. He repeatedly uses his intelligence and small size to protect the flock from their enemy, Smirre the fox. These actions signal an early shift from selfishness towards courage and loyalty. Further experiences—aiding distressed animals, witnessing natural wonders, and encountering diverse communities—foster empathy, responsibility, and respect for all living things.

Nils's relationships with the geese drive his development. His bond with Morten evolves from necessity into deep friendship, while Akka mentors him, challenging his prejudices. Nils consistently prioritizes the flock's safety over his own desire to return to human size, even refusing the tomte's offer to reverse the curse if it means abandoning Morten to danger. This self-sacrifice marks his full moral transformation. His journey culminates in Lapland, protecting the geese and reconciling with his past. Upon returning home, his newfound integrity is tested; he selflessly prevents Morten from being slaughtered. This act breaks the curse, restoring Nils to his original size. He retains the ability to communicate with animals, symbolizing his enduring connection to nature and the lessons learned.