The title character is Mary Louise Burrows. In this first book of the series, she is a fifteen-year-old girl with unusual maturity (though the other girls in her boarding school find her somewhat priggish). She is suddenly confronted with the fact that her beloved grandfather is suspected of no less a crime than treason against the United States. With the help of old and new friends of Mary Louise, her grandfather's innocence is revealed and the truth is uncovered. The novel features a federal agent named John O'Gorman; he is assisted by his daughter Josie, a young woman he has himself trained to function as an investigator.