In which Holmes pursues the elusive trail of a steam launch named the Aurora with the aid of his keen intellect, Toby the dog, and the eager eyes of the Baker Street Irregulars, while ruminating on the strange, diminutive footprint of a savage companion linked to a distant island. As the river’s mysteries deepen, Watson finds brief respite in the soothing strains of Holmes’s violin, pondering the curious case and its far-reaching ties.