The fourth Relentless movie has Detective Sam Dietz (Leo Rossi) investigating a series of ritual murders committed by another L.A. serial killer. The only clue linking the victims is that all of them were in the services of a shady and unconventional therapist named Dr. Sara Lee Jaffee (Famke Janssen) who may know a lot more than what she is telling. Dietz and his new detective partner, Jessica Parreti (Colleen Coffey), think there is a link between Dr. Jaffee's therapy sessions and the murder victims when they learn that all of them had a near-death experience in the past. On the home front, the recently widowed Dietz struggles to raise his teenage son, Cory (Christopher Pettiet), on his own and deals with the troubled teenager slacking off in school, abusing drugs, and carousing with a local and similarly troubled teen girl.