When brothers, Levi and Deevee, discover the existence of a cave under the small town of Willow City, North Dakota, they have to share it with their best friends, Gus and Mike. Together, the boys cannot resist the search for the elusive cave entrance that was hidden by the town's mayor years ago for safety reasons. Now, in the 1950s, finding it will take creative minds, a bit of sneakiness, and skills that small-town life helps build. But problems develop when an old Chippewa woman demands they stay away from the cave. Last seen fifty years ago, her knowledge of the site only intrigues the boys, especially, after Levi is frightened by her.
The middle-school-aged boys decide to press forward and begin the search for clues, but it will not be easy. As a series of events points to a location, they are forced to keep it a secret, too, for fear their plans will be thwarted by safety-conscious adults. The boys risk their lives figuring out the old Chippewa woman's secret which she will not fully reveal. The Willow City boys work together to find the cave, explore it, and solve the reason its entrance was hidden so many years before.