London, 1899. After spending time in warzones abroad, Winston Churchill considers a Parliamentary career. But a memento from his visit to Cuba, four years earlier, returns to haunt him. Across the city, the Great Detective has a mysterious caller, all the way from Havana. As ruthless mercenaries wield alien powers, young Winston and Madame Vastra learn they have a mutual friend - an eccentric young man, sporting a bowtie…