S6, E4: A jogger stroller and baby are found in Central Park with a riddle attached, giving the detectives till tomorrow to find the baby's mother. All kinds of clues follow but tragically, by the time the detectives find the mother, she's dead. The psycho kidnapper starts them on a new chase, and the initials RDK are the biggest clue. Cragen (Dann Florek) remembers that RDK was a serial killer from the late 70's early 80's who raped, kidnapped and dismembered his victims (thus the nickname). The question is, could RDK have resurfaced or could it be a copycat they're dealing with?