"World’s End Harem ~Britannia Lumiére~" is a spin-off of the "World’s End Harem" series, set in an alternate timeline where a virus has decimated the male population, leaving women to dominate society. The story unfolds in the United Kingdom, now called Britannia Lumiére, a matriarchal nation ruled by Queen Elizabeth III. The narrative centers on Reito Mizuhara, a young man who awakens from cryogenic sleep five years after the outbreak of the MK virus, which has rendered most men infertile. As one of the few surviving fertile males, Reito becomes a crucial figure in the government's efforts to repopulate the world.

Reito is placed under the care of the Britannia Lumiére government, which assigns him to a facility where he is expected to engage in reproductive activities with selected women. However, Reito resists these expectations, driven by his unwavering love for his childhood friend, Erisa, who remains in cryogenic sleep. His refusal to comply with the government's demands creates tension, as his unique status makes him a valuable asset in the struggle to restore humanity.

The series explores the political and social dynamics of Britannia Lumiére, where women hold all positions of power, and men are either rare or subjugated. Reito's presence disrupts the established order, leading to conflicts with powerful figures such as Queen Elizabeth III and her advisors, who view him as both a resource and a potential threat. The narrative delves into themes of autonomy, consent, and the ethical implications of using individuals as tools for societal survival.

Key characters include Reito Mizuhara, whose determination to remain faithful to Erisa drives much of the plot; Queen Elizabeth III, a pragmatic and calculating ruler focused on preserving her nation; and various women within the facility who interact with Reito, each with their own motivations and struggles. The story balances personal drama with broader societal issues, creating a layered narrative that examines the consequences of a world reshaped by gender imbalance and survivalist policies.