The book is about a young lad, Nils Holgersson, whose "chief delight was to eat and sleep, and after that he liked best to make mischief". He takes great delight in hurting the animals in his family farm. Nils captures a tomte in a net while his family is at church and have left him home to memorize chapters from the Bible. The tomte proposes to Nils that if Nils frees him, the tomte will give him a huge gold coin. Nils rejects the offer and the tomte turns Nils into a tomte, which leaves him shrunken and able to talk with animals, who are thrilled to see the boy reduced to their size and are angry and hungry for revenge. While this is happening, wild geese are flying over the farm on one of their migrations, and a white farm goose attempts to join the wild ones. In an attempt to salvage something before his family returns, Nils holds on to the bird's neck as it successfully takes off and joins the wild birds.

The Wonderful Adventures of Nils (orig. Nils Holgerssons underbara resa genom Sverige, Nils Holgersson's wonderful journey across Sweden) is a work of fiction by the Swedish author Selma Lagerl f. It was published in two books, The Wonderful Adventures of Nils in 1906 and Further Adventures of Nils in 1907. These two are usually combined into a single book called The Wonderful Adventures of Nils, although that name could also describe the first book only.

The background for publication was a commission from the National Teachers Association in 1902 to write a geography reader for the public schools. "She devoted three years to Nature study and to familiarizing herself with animal and bird life. She has sought out hitherto unpublished folklore and legends of the different provinces. These she has ingeniously woven into her story." (From translator Velma Swanston Howard's introduction.)

Selma Lagerl f, like many leading Swedish intellectuals of her time, was an advocate of Swedish spelling reform. When published in 1906, this book was one of the first to adopt the new spelling mandated by a government resolution on April 7, 1906 (see Svenska Akademiens Ordlista).