The Man Who Knew is a mystery novel by Edgar Wallace that explores the life of a wealthy, paranoid capitalist and his scientific secretary. The story begins in a laboratory, where the secretary is deeply engrossed in conducting an experiment while obsessively studying a photograph of a young girl. The capitalist, deeply concerned about his financial situation, becomes more anxious as he interacts with his secretary and a young woman, whose integrity seems to be central to the developing drama. The atmosphere is tense, with suspicions brewing, particularly regarding the capitalist's strange behaviors and the relationships unfolding between the key characters. The narrative delves into a world of intrigue, where deception and uncertainty begin to blur the lines of trust. As the plot develops, it hints at deeper mysteries, possibly involving crime and danger. As the tension rises, the reader is drawn into a complex web of secrets and lies, with the potential for betrayal, crime, and unexpected romantic undertones. The story sets the stage for an unraveling mystery, revealing that not everything is as it seems, and danger lurks in unexpected places.