The Aldens are preparing for a yard sale when they find an old wooden mask in the trash behind a neighbor's house. Violet notices that the mask looks like a Native American mask that she had read about. When the children go to the library to learn more about the mask, they discover that they have found an old Native American mask from the Hopi tribe. When the children learn there will be an Intertribal Pow-Wow at the park over the weekend, they plan to deliver the mask to its rightful owners.

But Henry, Jessie, Violet, and Benny soon find out that they're not the only people interested in the history of the old wooden mask. A strange man is following them around, and an antique dealer is looking for more than her fair share at the Boxcar yard sale. When the mask is stolen from their garage, the Aldens vow to find it before the weekend Pow-Wow. But with such little evidence, will they be able to find the mask in time?