In order to earn money to bring Fritz's momma and sisters from Sweden to America, the determined Fritz sets out to master a new skill, working as the youngest stonemason on the crew building the Stone Arch Bridge across the Mississippi River. Along the way, Fritz, always eager to learn, unlocks secrets of his new homeland from details of the river's geology and the area's animals to stories of brave stands against slavery and the dangers of flour milling.

He befriends Margaret, Metis girl whose family has called the region home for generations, and meets notable early settler Emily Goodridge Grey. Applying his developing stonecutting skills, he records these many lessons in the keystones of the bridge.

Working through the summer, Fritz grows in responsibility and stature. Best of all, soon after the bridge is complete, he is able to show off his handiwork to Mamma in person.