Assistant U.S. Attorney Zoe Landau -- Lightman's ex-wife and Emily's mother -- brings a case in. She wants Lightman to help investigate a serial arsonist case in which the only key eyewitness is a 5-year-old boy. While Zoe gives glimpses about what it's like being married to a human lie detector, it quickly becomes clear that she and Lightman still have feelings for each other.
Meanwhile, Foster and Torres are immersed in the world of rival rap groups when one rapper gets killed in a drive-by shooting.