Gabriel Book, played by Mark Gatiss, operates as an antiquarian bookseller in 1946 London whose concealed sexuality drives the personal storyline alongside the mysteries. His arrangement with childhood friend Trottie, portrayed by Polly Walker, unravels under pressure from her developing connection with Colonel Winters, played by Rupert Graves.

This setup functions as the core character premise rather than background detail, embedding the concealment of identity and external threats into the season's ongoing arc. Creator Mark Gatiss has stated an explicit intent to highlight historical LGBTQ+ oppression and the societal dangers of the postwar period, positioning these elements as foundational to the series. The queer themes integrate into the traditional cozy mystery format without direct lectures, yet they share narrative space with depictions of historical fascist elements across episodes. This deliberate foregrounding elevates identity politics to a primary driver, shifting focus from pure procedural storytelling to commentary on fragility of progress.