People who seem to have nothing in common are being killed in the Southeast USA. Grace's autopsy yields that all the victims has the same rare blood type. The team uncovers evidence that confirms all the victims were the children of Chris James Allmon, a Charles Manson-like cult leader currently incarcerated in a Georgia psychiatric facility. Sam interviews the adoptive parent of one of the remaining children and quickly surmises their son, Toby Watson, is the killer. She theorizes that he is killing his siblings to 'cleanse' himself of his deadly lineage. Meanwhile, Sam confronts her father, Walter Anderson, about his unethical experiments during the 1960s involving viral experiments that led to the mental breakdown of 34 people.