In short, this fourth novel in the series feels a bit dreary and overburdened with problems.  The lovely, orphaned Lady Isidore has been waiting eleven years for her estranged husband to return from his wanderings around the world.  Simeon (Duke of Cosway) reluctantly arrives home to take care of the beautiful, but feisty, Isidore and his parents’ crumbling estate, which has fallen apart from old age, unpaid bills, angry villagers, and shoddy workmanship—especially in construction of the water closets.  The raw sewage permeating their estate nearly becomes a character itself, with frequent references to the stench and plot points worked around the cleaning of the sewer.

A subplot involves Isidore’s friend Jemma, who fluctuates aomng her various feelings for her own gravely ill and long-estranged husband.  Their uncomfortable situations, and various affairs, remain unresolved at the book’s conclusion.  Simeon’s abusive mother also plays a part in the story until she thankfully takes up residence elsewhere due to problems surrounding the sewage from the water closets.  The story of Isidore, Simeon, and the water closets, however, reaches an eventual conclusion.  Surprisingly for a historical romance, When the Duke Returns is a bit depressing with only a few moments of intrigue (when Simeon discusses his travels) and passion (when Isidore and Simeon finally consummate their marriage) to brighten up the gloomy story of disease, death, dysfunction, and philandering.