The sudden arrival of a mysterious heir to the marquisate of Brotherton triggers a complex legal and social struggle that threatens the stability of the Germain family. The protagonist, Lord George Germain, finds his position and inheritance challenged by the unexpected appearance of his brother's son, the titular Popenjoy, who arrives from Italy with a foreign mother and questionable legitimacy. As the family navigates the rigid expectations of Victorian society, they must contend with the machinations of the current Marquis and the public scrutiny surrounding the claim. The narrative employs a third-person omniscient perspective to examine the interplay between private morality and public reputation.

Readers and critics often note that this novel serves as a sharp critique of the social anxieties prevalent in the mid-1870s. Discussion frequently centers on Trollope's ability to weave a sensational legal premise into a nuanced study of domestic life and character motivation. Many highlight the effectiveness of the author's dry wit and his keen observation of the rigid class structures that governed Victorian interactions. The balance between the central mystery of the heir's legitimacy and the broader social commentary is frequently cited as a hallmark of the work. Readers appreciate the depth of the character development, which prevents the plot from becoming merely a vehicle for social satire.