On the day of convicted serial killer-rapist John Mathers's execution, a new case following his MO causes a stay after countering the already administered lethal injection and urgent forensic (re-)examination. It was Catherine's first case leading to death row. The most modern techniques, even requiring federal equipment not available to Greg, show another killer was at work and probably one was a copy-cat, but the question remains in which order, as that may or not discharge Mathers.