London, 1678. New Year's Day. The body of a young boy, drained of his blood and with a sequence of numbers inscribed on his skin, is discovered on the bank of the Fleet River. With London gripped by hysteria, where rumours of Catholic plots and sinister foreign assassins abound, Sir Edmund Bury Godfrey, the powerful Justice of Peace for Westminster, is certain of Catholic guilt in the crime. He enlists Robert Hooke, the Curator of Experiments of the Royal Society, and his assistant, Harry Hunt, to help. Hooke and Hunt must discover why the boy was murdered. Moreover, what does the cipher mean?