Still feeling responsible for Don's near fatal stabbing, Charlie throws himself into FBI case work and discovers what he believes to be an unnoticed serial killer working in Los Angeles. His discovery relies heavily on complex mathematical modeling, and because of this David, who is in charge while Don recovers, can't justify committing resources to the case until Charlie has hard evidence. He instead teams Charlie up with Ron McGill (the conspiracy theorist from ep. 509), who introduces Charlie to amateur sleuths tracking serial killers. With their help, Charlie is able to discover two other locations where the serial killer had practiced and then moved on from, and ultimately discovers the killer's victim zero. With this additional information, Charlie is able to identify the killers M.O., and using the hot zone equation from the pilot, helps the team break up the killer's next attack. However, knowing that the killer has already moved two times, the team works quickly to bring him in before he moves on again. Using witnesses from the victim zero case in conjunction with Charlie's new discoveries, the team is able to track the murderer down and arrest him.
