An unemployed and seemingly dull-witted young man named David Grey stays with his friend Johnny Gilchrist in Liverpool while he looks for work. Due to a mistake, he is hired by a Spanish-owned shipping company whose owner "Don" Pedro de Guzman lives in a lonely, mysterious house in the marshes outside Liverpool. Grey's curiosity is aroused by the unexplained death of a man whom he had spoken to on the marshes, close to the house. His investigations lead to his abduction and detention on one of Guzman's steamers, which takes to Spain where he becomes embroiled in an attempted revolution in Spain, which fails. He is eventually able to escape thanks to the help of Guzman's daughter, Margarita.
