Devoted to the cause of the Union, 15 year old Joe Roberts finds life in a Vicksburg household intolerable despite a reunion with a brother and relatives, all southerners. After a harrowing escape aboard a Confederate transport ship, Joe makes his way to the Union Navy headquarters and signs up as a powder monkey for the gunboat Carondelet. Under the command of Captain Kerby and the special care of veteran Lew Remley, Joe participates actively in the bloody struggle between the Union and Confederate flotillas along the Mississippi. The capture of his uncle and brother awakens inner conflicts but their escape temporarily solves the problem. With the siege of Vicksburg, the Union achieves a vital stronghold in the area and Joe is content to have fought for his ideals. In a second meeting with his aunt, accompanied by Lew, Joe discovers the true identity of the veteran sailor (his missing father) and the mystery of his disappearance is cleared up. This last contrived note of melodrama but does not destroy the impact of an exciting, historically accurate adventure tale of spilt loyalties and courageous decision.