After two motorists die at the hands of a sniper, Danny and Kate are put on the case of what the city believes is a serial killer. They begin canvassing for clues and find some organic food bags near each shooting scene. Pairing the food bags with some eco-friendly bullets, and the fact that all the victims were driving gas guzzlers, gets them looking at radical environmental groups. Danny questions the head of one, a man named Leaf, who tells him the identity of the shooter. Danny and Kate arrive at the man's apartment to find him shot to death. They also see Leaf escaping out the back window, but Danny apprehends him after a short pursuit. Meanwhile, a bad bicycle accident sends Danny's son Sean to the hospital with a brain injury. Danny has a hard time dealing with his son's life-threatening injuries and relies on his job to get him through the tough time. Frank also has to walk a fine line with the Mayor about going public with the sniper in the city, and he, too, has trouble dealing with seeing his grandson in a hospital bed with an uncertain prognosis.