Karl Arnold works as an assistant in a clinic devoted to family counseling that largely caters to government employees. After his wife walks out with their two children Arnold uses his position at the clinic to stalk and eventually murder families that patronize the clinic. The BAU eventually stop Arnold.While in prison Arnold becomes involved in the case of another family killer, this time she is a young woman in a psychotic break reliving her hellish childhood and slaughtering military families. At the end of the episode Arnold reveals that the reason he became involved was at the prompting of the Boston Reaper to further torture and harass Agent Hotchner.