Bob Rowley of Hannibal, Mo., is the young hero of a story of riverboating in Mark Twain's day and with Mark Twain. Against his family's wishes, and full of cheek, Bob goes to work as ""cub"" under the Clemens he knew as Sam and must now call ""Sir"", and is taken down a peg or two while learning the ropes. Romance and politics enter the picture too- the former in the form of lovely Joyce Rennard of St. Louis and the latter with the forebodings of civil war. While Bob is politically undecided, Clemens and his superior, Capt. Bixby, hold strongly divergent views- the one pro South and the other pro North. Slowly Bob is drawn to the North. More quickly is he drawn to Joyce, but not without setbacks, for Mr. Rennard objects to Bob's forthright frankness and it takes some doing before Joyce is able to buck papa's will. All this-against the background of sidewheeler days and ways- presents a colorful picture of the times and is typical of Stong's light vein historical fiction.