The Eagle of the Ninth is both a mystery and the story of a young man whose growth to adulthood forces him to acknowledge painful truths about his background and his career. Marcus, a young centurion, begins his service in the Roman army of the second century with dreams of glorious military success and a triumphant return to the Etruscan farm that his family has owned for generations. Instead, he soon confronts feelings of alienation when his mother marries again and he is assigned to far-off Britain. Isolated from his family and country, Marcus must establish his own place and accept the realities of a sometimes harsh world.

The world of first-century Britain is depicted in vivid detail. The contrast between the peace and order of the Roman way of life and that of the Celts, who lived in more direct harmony with nature, is one of the major themes of the novel.