Porter arrives at the crime scene at 6:45 am The area is cordoned off with at least a dozen police vehicles, an ambulance, fire trucks, and press. A young officer directs him to Nash, who stands near a city bus with Tom Eisley from the Medical Examiner’s Office. The bus driver, Mr. Nelson, loudly insists the victim jumped in front of his vehicle and that he was not speeding. Porter reassures Nelson there will be no consequences if he cooperates. Nelson recounts that the light was green when a man suddenly leaped into the street. Porter notes a CCTV camera at the intersection. When Porter questions why Homicide was called for an apparent suicide, Nash reveals the victim was carrying a small white box tied with black string—the signature packaging of the Four Monkey Killer.

Porter reflects on the Four Monkey Killer case that has consumed five years of his life: seven victims, 21 boxes total, three for each victim. Nash clears the area to give Porter space. The victim has no identification, and his fingerprints yielded no match. Paul Watson, a CSI technician, approaches with a video camera to document the opening. Porter carefully unties the black string and unwraps the white paper, revealing a black box. Inside, resting on cotton, is a severed ear. The address label reads Arthur Talbot, 1547 Dearborn Parkway. Watson identifies Talbot as a wealthy investment banker and real estate developer involved in controversial waterfront projects.

Inside the tent at 7:05 am, Porter, Nash, Eisley, and Watson examine the body. The victim landed facedown, and when they turn him over, his face is destroyed—jaw broken, one eye ruptured. The right leg and left arm are broken in multiple places. Eisley says he can reconstruct an image of the face. The victim was wearing expensive shoes but a cheap suit, along with a fedora. In the breast pocket is a small composition book nearly filled with tiny, precise handwriting. They also find a generic dry cleaner receipt, a pocket watch stopped at 3:14, and 75 cents in change. Porter begins reading the diary. The first page identifies the writer as a murderer with an IQ of 156, known as the Four Monkey Killer (4MK).

The diary begins in a childhood flashback, where its narrator insists his parents are not to blame for what he became. His loving mother gave up her publishing career to stay home with him, and his father worked in finance and drove a 1969 Porsche. The family had a happy, regular routine and spent lots of time together. Even then, he had violent fantasies: he imagined their throats sliced open, and would laugh at the thought. He states this is when his dark tendencies began.

At 7:31 am, Porter and Nash arrive at the Talbot mansion. Nash reports the captain wants them back at headquarters, but Porter insists on pursuing the lead. The housekeeper, Miranda, answers through a security chain. Patricia Talbot, an attractive woman in her early forties, comes to the door. Assuming the detectives are there about her stepdaughter, she launches into complaints about Carnegie’s rebellious behavior: shoplifting, joyriding, drinking, and a recent overdose. She mentions her husband, Arthur, left early to play golf at Wheaton. They ask for Carnegie. Porter notes the teenager’s blonde hair, recalling that all but one of the Four Monkey Killer’s victims were brunettes. Her ears are intact.

As Porter and Nash rush back to their car in the rain, Nash reveals that someone tweeted a photo of the bus victim, and it has been retweeted over 500 times. Captain Dalton calls, furious. Porter argues they need to follow the Talbot lead, explaining that since the victim may be without water, they have at most three days to find her alive. The captain reluctantly gives them one hour. They speed toward Wheaton. A radio report details the Four Monkey Killer case, crediting Porter with naming him and describing the killer’s pattern: kidnapping victims to punish families for crimes, sending body parts every two days (ear, eyes, tongue), then leaving bodies with notes saying “DO NO EVIL” (28). Nash wonders why the killer did not take Carnegie if the pattern is to punish fathers through their children. Porter suggests someone check Arthur Talbot’s finances. Nash calls Matt Hosman from Financial Crimes to investigate, then begins reading the diary aloud as they drive.

The diary narrator describes the summer he turned 11, when Simon and Lisa Carter became his family’s neighbors. He then fondly describes his mother’s blond hair, green eyes, porcelain skin, and slender figure. Many admired her. He recalls overhearing his mother tell Mrs. Carter that she wore short dresses because her legs were her best asset and that the diary narrator’s father enjoyed them.

At the Chicago Golf Club, Porter and Nash argue with the manager until the receptionist drives them in a golf cart to Talbot. They interrupt Talbot’s game with the mayor and three business partners, including Louis Fischman, a former district attorney who worked the Elle Borton case, another Four Monkey Killer victim. Porter informs Talbot that a package containing a human ear was addressed to his home but both Carnegie and Patricia are safe. Porter explains the killer’s pattern of targeting families. Talbot makes frantic, unsuccessful phone calls. Porter correctly deduces Talbot has another daughter. Talbot confesses: His 15-year-old daughter, Emory Connors, lives at Flair Tower with a caregiver. Her mother, Catrina, died of a brain tumor when Emory was three. Talbot admits he has no photo of Emory for fear of Patricia discovering her.

Nash drives aggressively back to Chicago, enjoying Porter’s car. He reports texting the captain an update. Porter wonders how Talbot could hide a child for 15 years in the modern age, with social networks and constant press coverage. Nash suggests money can hide many things. Porter returns to reading the Four Monkey Killer diary.

The diary narrator describes visiting a lake in the woods behind his house, where he would sit beneath a large oak tree. A month earlier, he killed a stray cat and left its body by the lake to observe decomposition. After only a month, nothing remained but bones, leading him to conclude a person would disappear just as quickly. One June day, he secretly watches Mrs. Carter swim naked at the lake. The next morning, he overhears his mother and Mrs. Carter in the kitchen discussing sex. Mrs. Carter mentions being tied up for the first time and complains about her husband Simon’s drinking and erectile dysfunction. The narrator’s mother reassures her she is beautiful. When the narrator peeks around the corner, he sees his mother and Mrs. Carter kissing.

Emory Connors awakens at 9:29 am in total darkness with a severe headache. She briefly dreams her deceased mother is waking her for school, then realizes she is in an unfamiliar place. Her hand finds a thick, wet bandage where her left ear should be. She remembers being abducted in a park by a man claiming to search for his lost dog. He injected her with a sedative. She is naked and handcuffed by her right wrist to a hospital gurney. Her left hand is free. Fighting dizziness and nausea, she sits up. When she touches the bandage, she discovers with horror that her ear is gone, only a ragged, stitched wound remaining. She begins sobbing.

At 10:04 am, Porter and Nash arrive at Flair Tower Penthouse 2704, where officers and Watson are already investigating. Watson reports the apartment is spotlessly clean with no sign of struggle. Recent incoming calls were all from Emory’s father. Porter walks through the apartment, noting its sterile, staged quality. He finds a photo of a woman with green eyes he assumes is Emory’s mother. Watson discovers homeschooling textbooks. In a second bedroom, they find numerous framed photos of Emory, including one from a homecoming dance with a boy identified on the back as Ty. Watson enters, saying he has permission to continue assisting them with the case. The room contains large women’s underwear and a warm towel in a laundry basket, suggesting an adult housekeeper recently fled. A damp toothbrush in the primary bathroom confirms someone was there very recently. They conclude the housekeeper was warned of their arrival and left. Eisley calls, requesting they come to the morgue immediately.

The diary narrator details the next morning, when his mother serves Mrs. Carter coffee with whiskey before inviting her to the bedroom to try on dresses. The diary narrator goes outside and watches them through the bedroom window. He sees the women drinking whiskey directly from the bottle. Mrs. Carter undresses to her underwear to try on a dress. The diary narrator’s mother steps behind her, kisses her neck, and begins caressing her breast. In the mirror’s reflection, his mother’s eyes meet his, but she still begins having sex with Mrs. Carter.

At 10:31 am, Porter and Nash meet Eisley at the Cook County Medical Examiner’s Office morgue. Eisley reveals the victim had advanced stomach cancer and only weeks to live. A toxicology screen showed several medications, including trastuzumab, an intravenous cancer drug administered only at hospitals or specialized treatment centers. The man still has a full head of hair, suggesting he was not undergoing chemotherapy. Eisley shows them that the expensive shoes are nearly two sizes too big; the victim had stuffed tissue in the toes. Nash successfully lifts fingerprints from the shoes. Eisley points out a small, fresh tattoo on the man’s inner right wrist resembling the number eight or an infinity symbol. He mentions a friend at the Museum of Science and Industry who may be able to reconstruct the victim’s face from his skull.

As the diary narrator stands outside, Mr. Carter’s car arrives home unexpectedly. Mrs. Carter panics when she sees her husband and quickly leaves. The narrator hears a loud slap and scream from the Carter house. Mr. Carter storms out, pushes past the diary narrator, and pounds on the diary narrator’s back door, shouting obscenities at his mother. Mrs. Carter appears in her doorway with a bruised, swollen eye, begging the diary narrator not to let Mr. Carter hurt his mother. The diary narrator sees his mother watching from a window, then motioning for him to stay put. Mr. Carter threatens both women. The diary narrator’s mother opens the door and beckons Mr. Carter inside. The diary narrator hears a horrible scream that is not his mother’s voice.

At 9:31 am, Emory fights back vomit and begins to process her situation. She realizes her severed ear is part of the Four Monkey Killer’s signature pattern. After intense sobbing, she examines the handcuffs and finds no release mechanism. She knows the killer’s pattern: ear, then eyes, then tongue. An inner voice that sounds like her deceased mother taunts her. The voice suggests she explore the room. Despite the pitch darkness, she forces herself to stand and begins moving along the walls, dragging the gurney behind her. The walls are rough cinder blocks. The voice suggests her captor may be watching via infrared cameras. After exploring the perimeter, she realizes with horror that the room has no door. Suddenly, a loud click sounds from above, and music blasts down at deafening volume—Mick Jagger singing about the devil.

At 11:30 am, Porter enters the task force room in the basement of Chicago Metro headquarters. He tries calling Heather again, reaching only voicemail. Detective Clair Norton arrives, followed by tech expert Edwin Klozowski and Watson. Porter briefs the team on the Four Monkey Killer case history. Watson notes the killer’s escalating timeline and questions why Barbara McInley, the only blonde victim, deviates from the pattern. He also notes that the killer always chose a daughter to kidnap despite several families having sons.

They review what they know about Emory Connors. Klozowski uses data from Emory’s Fitbit to locate her running route at A. Montgomery Ward Park. He reveals he hacked boyfriend Tyler Mathers’s Facebook and found the boy’s contact information. Building elevator logs show someone named N. Burrow, presumed to be the housekeeper, left the penthouse minutes before police arrived. Porter summarizes the morgue findings: advanced cancer, ill-fitting shoes, a stopped watch, and the diary.

He assigns tasks: Klozowski will track the dry cleaner receipt and CCTV footage, Norton will check the park and cancer centers, Watson will consult his uncle (an antiquarian) about the watch and research Emory’s mother, and Porter and Nash will question Tyler Mathers. Nash reads the diary’s taunting final page aloud. The team debates whether the killer committed suicide or was creating a legacy. They wonder what his death means for Emory’s chances of survival.