One day in April 1982, Wade Davis, a Harvard ethnobotanist and plant explorer, stepped off the plane in Port au Prince, Haiti, to investigate two documented cases of zombis - people who had reappeared in Haitian society years after they had been officially declared dead and had been buried. Davis had been sent by a team of medical researchers who believed they zombification was caused by a powerful sedative, a drug that might revolutionize surgical anaesthetics. His mission was to find the drug.