No disease sparked as much dread in the 18th century as smallpox. The virus killed millions across the globe, carrying off one in every five victims and disfiguring survivors with tell-tale pitted scars. But one mysterious method offered hope: inoculation, the practice of piercing the skin to insert a drop of infected matter and induce a mild case of smallpox. Those inoculated safely were protected from the ‘Speckled Monster’ and stayed immune for life. Only one problem remained: convincing people to accept the treatment. Arguments raged in Europe over its risks and benefits, and public resistance ran high. In Russia, Catherine the Great took a momentous decision: to undergo inoculation herself to set an example in her empire and beyond. In 1768, she summoned the Quaker physician and inoculator Thomas Dimsdale from his Hertfordshire surgery to her glittering court in St Petersburg to carry out the secret mission that would transform both their lives. This is a fascinating history of Enlightenment ideals, female leadership and the fight to promote science over superstition.