When a mysterious mosquito-borne virus wipes out a community of indigenous Hawaiians on the tiny Pacific island of Ni'ihau, it has all the hallmarks of a field test of a state-sponsored biological weapon on unwitting human guinea-pigs. Eager to make his mark, the director of MECCAR's new sister centre in Indonesia solicits Giles's help to identify the perpetrators. Fiona's laboratory skills show it to be a Mexican strain of the West Nile virus, a common and usually innocuous species, engineered to carry a permanently active form of the Achilles gene. Having spotted Stephen Salomon at a virology congress in Mexico City when en route to Jakarta, she and Giles waste no time retracing their steps. Molecular analysis of a blood stain from a swatted mosquito in a ship's cabin, a day trip to the Maya ruins of Palenque, and an encounter with a deserted Mercedes Benz sow the seeds for a 3000 km road journey through rainforest, mountains, and desert in pursuit of the suspects. At the end of their journey, as they follow the flight path of a private jet from Tijuana on Fiona's smartphone, they know they must act swiftly and boldly if they are to avert a humanitarian disaster.