When a Milwaukee helicopter pilot/traffic reporter goes blind suddenly in the middle of a broadcast and crashes his helicopter, Hood and Rachel are called in to investigate this high profile mystery. Hood studies broadcast tapes and discovers the victim had been experiencing subtle symptoms for the two weeks leading up to the crash. When the pilot's pregnant wife also experiences sudden blindness, Hood can test his theory that the problem is mercury poisoning. When 25 other victims turn up, the race is on for Hood and Rachel to find the source of mercury poisoning before the damage grows and becomes irreparable. He discovers that cows fed certain food from a local fishery have been producing mercury laden milk and must solve the seemingly impossible task of finding where the mercury is in Lake Michigan and how its getting there in order to stop the crisis. Turns out to be a local conservationist with a twisted sense of how to make the world notice that they're polluting the environment.