The book begins in the winter of 1916. Vaino, a young Finnish boy, lives with his mother, brother and sister (I don't recall the location of the father being either explained or significant, but I could be wrong about this) in Russian-occupied rural Finland. Russian soldiers are often on the lookout for members of the resistance, and many elements of Finnish culture, including the spoken language, are forbidden.
Vaino's mother often helps pass the time with folk tales of Vaino's namesake, a Finnish god named Väinämöinen, who helps landscape the earth after its creation, and then goes on quests in it (often with his friend Imarinen, a blacksmith of legendary talent).
Over time, Vaino becomes more invested in the resistance movement as his mother ingrains love of his country in him, his brother joins the resistance, and his sister leaves home to marry a member of the resistance on the fly, far from home. His family also wins the support of a Russian soldier by assisting him generously in his time of need. This soldier later defects in favor of the Finnish side and is of great use to the resistance. Vaino also comes of age as he finds his place in fighting for Finnish freedom.
It seems that recently there have been a number of stories of people in love with their homeland, living under the oppression of outsiders, and fighting to free the country they care for. I wish I had a better understanding of what, exactly, was going on in the world at that time to make this kind of storyline so appealing to American audiences, as World War I was well over and done with by then. All Quiet On the Western Front won the Academy Award that year; perhaps it captured imagination enough that parents wanted something similar for their children, only a bit happier.