In the sequel to The Fourth Monkey, a new serial killer roams the streets of Chicago as Detective Sam Porter digs ever deeper into the dark past of the Four Monkey Killers.

Detective Porter and his team have been banned by the FBI from hunting the Four Monkey Killers. When the body of a young girl is found in the frozen waters of Jackson Park's lagoon, the police quickly discover that it is Ella Reynolds, who disappeared three weeks earlier. But how did it get there? The water in the lagoon has been frozen for months. Moreover, this is not the only strangeness of the case. Ella Reynolds is wearing the clothes of another girl who disappeared two days earlier. While the detectives of the Chicago Police Department do everything they can to solve the case rich in twists and turns, Porter secretly pursues the Four Monkey Killers. He knows for a fact that the easiest way to track him down is to find his mother. When the captain learns of Porter's recreational activities, he suspends the detective and his companions, Clair and Nash, must find the new killer on their own.

Porter obsessively pursues his target. He follows a grainy photo from Chicago to New Orleans, encountering a dark world beyond his imagination. He also realizes that there is only one place more terrifying than the mind of a serial killer: the mind of his mother who gave birth to him.