After a nearly 20 year absence, the notorious San Francisco serial killer, ""Zodiac,"" returns to taunt Nash as well as haunt Nash's mentor (who has since retired). But is it the actual Zodiac, or just a lethal copycat? Meanwhile, Joe goes undercover as a bike messenger to nab a free wheeling drug dealer.

Nash and Joe offer freedom to gangland money launderer Carlos Casas if he will testify against others.