Spring 1927. The birth of popular music. John Coughlin is a song-catcher from New York who has been sent to Appalachia to source and record the local hill-country musicians. His assignment leads him to small-town Tennessee where he oversees the recording session that will establish his reputation. From here he ventures further south in search of glory. He is chasing what song-catchers call the big fish or the firefly; the song or performer which will make a man rich.

Waylaid at an old plantation house, Coughlin gets wind of a black teenage guitarist, Moss Evans, who runs bootleg liquor in the Mississippi Delta. The Mississippi has flooded, putting the country underwater, but Coughlin is able to locate the boy and bring him out. Coughlin views himself as a saviour. Others regard him as a thief and exploiter. Coughlin and Moss the catcher and his catch pick their way across a ruined, unstable Old South and then turn north through the mountains, heading for New York.