HMS Nautilus is on patrol off the west coast of Africa, intercepting the American slave ships that were trying at that time to purchase cargoes of slaves from the dealers, and then to take them across the Atlantic in loathsome conditions. Slavery had been abolished in British territories in 1772, many years before, and the British were actively policing African waters in the hope of deterring the Americans and the Portuguese from retaining the slave trade. Nautilus has two midshipmen aboard, and one of these, Mark Vandean, is the hero of the story. The book is in the usual Manville Fenn style, with a succession of dreadful situations in which the hero finds himself. ''How ever does he extricate himself from this?'' the reader is continually asking. Of course he does, but it is often by means of something quite unexpected. A Black Bar is a device in heraldry, indicating that there is something shameful in the wearer's ancestry. According to Wikipedia: ''George Manville Fenn (January 3, 1831, Pimlico - August 26, 1909, Isleworth) was a British writer. He worked as a teacher in Lincolnshire, until he became printer, editor and publisher of various magazines. He had eight children with his wife Susanna Leake, whom he had married in 1855. Most of his work consists of adventure stories for young readers, featuring Explorers, Smugglers, young Adventurers and Seamen. His adult novels offer critical social commentary on Victorian England, especially reconsidering economic questions.''