A traveler journeys to a remote Transylvanian region and becomes trapped by an aristocratic outsider whose nocturnal predations extend to England. The plot follows how friends, family, and specialists piece together diary entries, letters, and newspaper reports to diagnose and pursue the menace, tending to a victim afflicted by its influence. The narrative uses an epistolary structure to shift perspective and build suspense, and explores tensions between modern science and ancient superstition, anxieties about foreign intrusion and contagion, and repressed desires, while culminating in a transnational pursuit back to the source of evil.