J. D. Barker’s The Fourth Monkey (2017) is a serial killer thriller and the first installment in the 4MK Thriller series. The novel follows Detective Sam Porter of the Chicago Metro Police, who has been hunting the elusive Four Monkey Killer (4MK) for five years. When a man believed to be the killer dies in a bus accident, Porter discovers a diary on the body. This discovery launches a high-stakes investigation as Porter must decipher the killer’s twisted history to find a final victim who may still be alive. The novel explores themes including The Familial Inheritance of Violence, The Corrupting Nature of Vengeance, and The Manipulation of Narrative and Identity.

An international best-selling author, Barker is known for his work in the horror and thriller genres. His debut novel, Forsaken (2014), was a finalist for the Bram Stoker Award. His reputation was further established when the Stoker family estate selected him to co-author Dracul, an official prequel to the classic 1897 novel Dracula. The Fourth Monkey was followed by two sequels, The Fifth to Die (2018) and The Sixth Wicked Child (2019), which continue the story of Detective Porter and his antagonist. Shortly after its publication, film rights for The Fourth Monkey were acquired by CBS Films.

This guide refers to the 2018 Mariner Books paperback edition.

Content Warning: The source material and guide feature depictions of graphic violence, child abuse, physical and emotional abuse, animal cruelty and death, illness and death, death by suicide, sexual content, and cursing.

Plot Summary

Sam Porter is a Chicago homicide detective on personal leave after his wife was accidentally killed in an armed robbery at a corner store. He is called back to duty by his partner, Detective Nash, to investigate a bus accident. The victim intentionally jumped in front of the bus. At the scene, they find he was carrying a small white box tied with black string, the signature of the Four Monkey Killer, or 4MK, a serial killer Porter has pursued for five years. The victim is assumed to be 4MK, as he also possesses      4MK’s diary detailing his childhood.

The box contains a human ear and is addressed to Arthur Talbot, a wealthy real estate developer. The victim’s other possessions include a stopped pocket watch, a dry cleaner’s receipt, and expensive shoes too big for the victim. 4MK tortures and kills the children of people who’ve committed crimes, so Porter and Nash visit Talbot’s mansion, where they meet his wife, Patricia, and stepdaughter, Carnegie. Carnegie is unharmed, confirming she is not the victim. The detectives find Talbot, who reveals he has a secret, illegitimate 15-year-old daughter, Emory Connors, whom he believes is 4MK’s latest victim. Amid the investigation, Porter begins reading 4MK’s diary.

The first diary entry describes a seemingly perfect childhood but reveals a dark nature, admitting to killing a cat to watch it decompose and having violent fantasies about his parents.

The narrative shifts to Emory, who awakens naked and disoriented in a dark room, handcuffed to a hospital gurney. Her ear has been removed. She recalls being abducted in a park by a man who claimed to be looking for a lost dog. During her captivity, she is tormented by hours of loud music and hallucinates conversations with her deceased mother. Her attempt to find water results in the gurney toppling, breaking her wrist.

Meanwhile, the diary entries detail 4MK’s childhood, including the arrival of new neighbors, Simon and Lisa Carter. The diary narrator becomes obsessed with Mrs. Carter, witnessing her skinny-dipping and later discovering a sexual relationship between her and his own mother.

Porter and Nash search Emory’s penthouse apartment. They discover a photo of Emory with her boyfriend Tyler Mathers and learn she is homeschooled by a live-in tutor, Nancy Burrow. Meanwhile, the medical examiner reports the bus victim had terminal stomach cancer.

The narrative returns to the diary, where its narrator describes how Mr. Carter discovers the affair and brutally beats his wife. Mr. Carter then confronts the diary narrator’s mother, who lures him into her house. Later, the diary narrator and his father find Mr. Carter chained and tortured in their basement, where the diary narrator’s mother and father ultimately kill him.

The official 4MK task force reassembles, including Porter, Nash, Detective Clair Norton, and tech expert Edwin “Kloz” Klozowski. CSI technician Paul Watson joins as a consult. The team reviews 4MK’s pattern of punishing family members for their “evil” deeds. Clair investigates the park where Emory disappeared and finds witnesses who saw a man carrying an unconscious Emory away. When Nancy Burrow returns to the apartment, she points out a calculus textbook that mysteriously appeared days earlier.

The book’s publisher address leads the team to a condemned warehouse, Mulifax Publications. The raid leads to a major breakthrough: In the rat-infested subbasement, they find the decomposing body of a man handcuffed to a gurney. The victim holds a brochure for The Moorings Lakeside, a Talbot development. The team discovers old bootlegging tunnels beneath the building. Porter follows one tunnel and finds three more of 4MK’s signature white boxes containing the victim’s ear, eyes, and tongue. The tunnel emerges at the Moorings construction site.

The diary narrator disposes of Mr. Carter’s dismembered body in the lake. So she won’t report them to the police, Mrs. Carter is held captive and tortured; however, after mysterious men start looking for the Carters, they learn Mr. Carter was embezzling funds from a criminal group for which he worked.

The investigation accelerates with new revelations. The body in the warehouse is identified as Gunther Herbert, Talbot’s CFO. Talbot’s fingerprints are found on the shoes worn by the bus victim. A financial investigator reveals Talbot is running a massive fraud scheme connected to the Moorings, which is being built on land owned by Emory. The bus victim is identified as Jacob Kittner, Tyler Mathers’s uncle.

Under questioning, Tyler confirms his uncle Jacob isn’t 4MK. He confesses that a mysterious man paid his dying uncle to pose as 4MK and commit suicide in exchange for funding Tyler’s college education. Tyler’s role was to date Emory, steal Talbot’s shoes, and plant the calculus book. Soon after, Kloz discovers that Paul Watson’s CSI credentials are a sophisticated fake; a fingerprint from a railcar in the tunnel identifies him as Anson Bishop. Kloz warns Porter, but Bishop is already with him at his apartment. Bishop stabs Porter in the leg and escapes.

In the diary, the two men looking for the Carters want financial documents Mr. Carter stole proving the group’s crimes, as well as the money he stole. Despite the diary narrator’s family’s attempts to hide Mrs. Carter and their actions, the men continue to harass them.

Clair and Nash raid Bishop’s staged apartment, finding a box of financial ledgers proving Talbot’s crimes and Polaroids of a naked Emory. A note suggests Bishop intends to kill Talbot. They rush to Talbot’s house, finding the police guard dead and message written in blood on a wall. Inside, they find Patricia and the housekeeper tied up; Bishop has abducted Arthur Talbot. Meanwhile, Porter, recovering in the hospital, receives a taunting call from Bishop. Porter realizes the items on Kittner’s body were clues: the 75 cents for a specific parking meter, the watch’s time of 3:14 for an address, and the receipt for the adjacent dry cleaner. He leaves the hospital and goes to 314 West Belmont, a Talbot construction site. He finds Emory’s running clothes at the cleaner next door.

In a final confrontation in the diary, one of the criminals, Kirby, kills the other, Briggs, but not before Briggs kills the diary narrator’s father. The mother abandons the diary narrator, escaping with Mrs. Carter and Kirby. He realizes his mother was planning this crime the entire time and split the stolen funds with her conspirators. After, men from Talbot’s company arrive to investigate—Talbot was the head of the criminal enterprise Carter worked for, hence why 4MK is targeting him. The diary narrator burns the house down with Talbot’s men inside and enters the foster care system.

Inside the construction site, Porter follows a trail of body parts and bloody messages up the emergency stairs. On the 11th floor, he confronts Bishop, who is holding a tortured Talbot hostage near an open elevator shaft. He’s removed Talbot’s eyes and tongue; the body parts that Porter saw along the way belong to Talbot, not Emory. Bishop reveals Emory’s mother was Lisa Carter, who seduced Talbot as part of a long-term plot to inherit his fortune. Bishop then pushes Talbot down one elevator shaft. As SWAT arrives, Bishop steps backward into another shaft, rappelling into the tunnels below to escape. Just before passing out from blood loss, Porter hears Emory’s weak screams from a third elevator shaft.

Emory is rescued alive but severely dehydrated and injured. Porter recovers in the hospital, where he learns that Harnell Campbell, the man arrested for his wife Heather’s murder, has been released on bail. A few days later, back in his apartment, Porter finds a final white box on his bed. Inside is Harnell Campbell’s ear, identified by a distinctive tattoo. A note from Bishop implores Porter to help find Bishop’s mother.