For over five years, the Four Monkey Killer has terrorized the residents of Chicago. When his body is found, the police quickly realize he was on his way to deliver one final message, one which proves he has taken another victim who may still be alive. As the lead investigator on the 4MK task force, Detective Sam Porter knows even in death, the killer is far from finished. When he discovers a personal diary in the jacket pocket of the body, Porter finds himself caught up in the mind of a psychopath, unraveling a twisted history in hopes of finding one last girl, all while struggling with personal demons of his own. With only a handful of clues, the elusive killer's identity remains a mystery.