"The Horseman on the Roof" is a historical novel set in 19th century France during a cholera epidemic. The story revolves around an Italian nobleman who is a member of a secret revolutionary group. He is forced to flee his homeland due to political turmoil and ends up in Provence, where he encounters and falls in love with a beautiful woman. As he travels through the plague-ridden countryside, he faces various trials and struggles, reflecting on the nature of honor, duty, and love.

The book explores the 1854 cholera outbreak in London, focusing on the efforts of Dr. John Snow and Reverend Henry Whitehead to trace the source of the epidemic. Through meticulous investigation and mapping, they identify a contaminated water pump on Broad Street as the outbreak's origin, challenging prevailing beliefs about disease transmission. The narrative highlights the intersection of urban development, scientific inquiry, and public health, illustrating how their groundbreaking work laid the foundation for modern epidemiology and transformed urban sanitation practices.