One out of every three fires in the United States is set deliberately. Firefighters must report the cause and origin of each fire they fight. If the cause of a fire cannot be determined immediately, a fire investigator may be assigned to the case. Investigators sift through the remains of a fire for clues about its cause, and if there is evidence of arson, the investigators also collect information to help police identify and locate the arsonist. In this episode of NOVA, a series of similar arson fires in California raises concern that a serial arsonist might be at work. The program follows the investigative team that solved this incredibly difficult case.