Dr. Theo Cray is on the hunt for a killer nurse, and redemption, in a mind-bending psychological thriller by the Wall Street Journal bestselling author of The Naturalist. Dr. Theo Cray has a legendary mathematical knack for catching serial killers. Until his exposure to a mind-altering pathogen knocks him off his game. It has upended an investigation, destroyed his reputation, and left him to question his own sanity. One person still trusts him to finish the job. His former professor Amanda Paulson is helping point Cray down a logical path to his prey: a nomadic health-care worker whose murder spree stretches back decades and whose victims number in the hundreds.