Tom and Kate, whose parents are missionaries in Africa, spend the summer in the home of a bachelor uncle, Mark Grey, on the Nez Perce Indian Reservation in northern Idaho. And what a summer it turns out to be! Uncle Mark is working on a research paper for the university, a key item of which is the Jefferson Peace Medal. (some of these rare medals were given to friendly Indians by Lewis and Clark during their expedition west. One of these medals has been loaned to Uncle Mark for his research, but just before Tom and Kate arrive, it mysteriously dissappears. Eventually it is found, but not before a weird string of happenings wich keeps things hopping all summer long.