Twelve-year old Caleb Wallace has spent his summer at Autumn Springs, coaxing his papa out of his room when he's grumpy, attempting to gain the affections of a certain nurse, and pulling himself together when his grandpa looks right into him with nothing but a dull stare.

Seems the only thing that puts a light in Papa Clem's eyes is a family heirloom - an old folktale involving Robert Johnson's harmonica down in Arkansas. Caleb is happy to listen; two things Papa Clem can still do well are tell a story and play the blues. But when Caleb finds his grandfather all spruced up with his guitar slung over his shoulder, set to take a trip down to the bus station, Caleb isn't sure if it's real or just the dementia talking.

Caleb decides to tag along, and what begins as a joyride around town becomes a three-state drive into time. Caleb and Clem endure bus trouble, layovers, and even play for tips under the Nashville skyline. Caleb, who's never left his hometown, knows he should pull the plug on this journey but can't force himself to make the call. Weary and penniless, the traveling bluesmen step off the bus and into muggy hot Mosby, Arkansas, ready to go looking for Robert Johnson's harmonica. And what they find changes not only Caleb's summer, but his entire life.