Historical documentaries come no goofier or more compelling than American Interior. All it takes to tell the story of John Evans, late 18th-century Welsh explorer of northwest America, is a rock-singer/raconteur and his costumed dummy. The first is Gruff Rhys, our bearded, deadpan-droll story guide. The second is “John Evans”, his Muppettish companion: felt-formed features, wind-tossed ringlets, buckle shoes. Both radiate the sweet-natured derangement of the innocent abroad. There are encounters with Mississippi locals, with a garrulous Welsh professor bandying Lévi-Strauss truths in the land of Levi Strauss trews (“Myth itself can become an operative historical reality”). There are maps, cartoons, lantern slides. It’s high entertainment, like Mark Twain gone multimedia.