Holmes and Watson find themselves on the hunt for a serial killer after a murder rocks their inner circle. Also, Holmes' neurological recovery regresses and his sobriety wavers as his commitment to catching the killer grows into an obsession.

Holmes' former sobriety sponsor, Alfredo, asks for his help with an illegal endeavor in order to save his brother. Also, Holmes and Watson investigate the poisoning of a biology professor who was working on a secret genetics project prior to being killed.