As night falls and the museum empties, a lone night-shift guard makes his rounds, unsettled by the exhibits. He slips into a basement stairwell to smoke marijuana in an interior courtyard, then heads back up. Halfway up the stairwell, he hears the courtyard door slam below as a strange, rotten smell fills the air. Near the top landing, he senses swift movement behind him and is struck by a powerful blow that disembowels him. His last sight is of his own entrails on the stairs.

Bill Smithback waits in Lavinia Rickman’s office while she reviews his manuscript for the museum’s exhibition book. She strikes out a section on Aztec human sacrifice, condemning it as too graphic. She then objects to his coverage of the failed Gilborg expedition, insisting that the museum’s failures have no place in the book. She lays down three rules: No controversy, nothing to offend ethnic groups, and nothing to harm the museum’s reputation. She then demands that Smithback identify his anonymous sources. He refuses, citing journalistic ethics. Before she can respond, her phone rings, and as Smithback leaves, he overhears her exclaim in shock, “Not another!” (75).

Lieutenant Vincent D’Agosta is led to a dim museum stairwell, where a mutilated guard’s body lies at the bottom. D’Agosta berates the guard for tracking blood and interfering with the crime scene. When the crime-scene team arrives, D’Agosta issues orders for evidence collection. Special Agent Pendergast, a tall, slender FBI man in a black suit, arrives and introduces himself. When lights illuminate the carnage, D’Agosta vomits at the sight, then resumes giving instructions to his team. Pendergast notes the blood spatter on the wall and suggests conducting a ballistic analysis to determine the angle and force of the attack. D’Agosta accepts his suggestion with growing respect.

At the command post, Pendergast explains that he is from the New Orleans field office and has been tracking a series of unsolved murders with the same method: Victims found with their skulls opened and brains removed. He proposes leading the case and working collaboratively with D’Agosta, who agrees.

Returning to the scene with Security Director Ippolito, Pendergast deduces that the murder victim went to the stairwell to smoke marijuana; he points out a discarded joint in the courtyard. In a hallway beyond, he finds fresh blood smeared on a painting, proving the killer passed through. At a copper-sheathed door to the secure area, Pendergast observes fresh dents and deep claw marks gouged into the wooden frame, then forces Ippolito to retrieve the key to the restricted space.

Margo Green visits the museum herbarium to request plant specimens from Bailey Smith, an elderly, ill-tempered curatorial assistant. Assistant Curator Gregory Kawakita, Margo’s rival and another of Dr. Frock’s protégés, enters and encourages Smith to gossip about the recent murder. Smith shares rumors that a creature has long inhabited the museum’s basement, feeding on rats. He mentions that a curator named Morrissey vanished five years ago after a failed Amazon expedition and claims that the man’s mutilated body was later found in the basement. He adds that a guard from the metal shop also claimed to have seen the creature. Kawakita grows amused, implicitly dismissing the stories, then attempts to jump ahead of Margo’s specimen request.

D’Agosta and Pendergast wait in a curator’s study for Museum Director Winston Wright, who arrives with Ippolito. Pendergast deliberately ignores Wright, then takes forceful control of the situation by accusing Wright of subtly obstructing the investigation. He lays down three conditions for keeping the museum open. He insists upon complete staff cooperation and a daily curfew limiting access to the hours between 10:00 am and 5:00 pm, and he also reserves the right to close the museum on short notice. When Wright protests, Pendergast reaches for the phone to call the attorney general, and Wright capitulates.

Pendergast briefs Wright on the murder, showing graphic crime-scene photographs and explaining that according to the blood spatter analysis, the man was disemboweled by a powerful downward strike. He identifies Dr. Frock as the museum’s top anthropology curator and warns that one more death will result in the museum’s immediate closure, regardless of the Superstition exhibition’s scheduled opening that Friday.

In a genetic testing laboratory, technician Lewis Turow analyzes a DNA sample from the New York Crime Lab, attempting to match it to various big cat species. The results show very low feline matches but a 45.2% match to the human outgroup control, and significant matches to two gecko species. Assuming that the bizarre data is an April Fools’ prank by his supervisor, Buchholtz, Turow writes a joke report identifying the sample as coming from a fictional “Gecko-man.” He later finds Buchholtz waiting in the lab, not laughing.

Margo meets with Dr. Frock, who grudgingly outlines the new museum security measures and then suggests that she use Kawakita’s genetic sequence extrapolator program to analyze her plant specimens. Pendergast calls and summons Frock to the basement, so Margo wheels the professor there, finding a police barricade. Although annoyed by the interruption, Frock is impressed by Pendergast’s knowledge of his evolutionary theories, growing mollified enough to listen as Pendergast shows him a latex cast of a claw fragment found embedded in the body of one of the murdered children. Frock identifies the morphology as dinosaurian rather than mammalian, noting that all mammalian predators have five digits. Pendergast suggests the killer is using a primitive artifact fashioned from animal parts, but Frock notes the absence of a broken claw in the new crime scene, which would imply that two such weapons exist; Frock believes this explanation to be improbable.

Deputy Director Cuthbert arrives, and the group enters the secure area, where Pendergast shows them crates from the Whittlesey expedition to South America—an ill-fated journey that ended with some members killed in the jungle and the rest dying in a plane crash. One damaged crate sports claw marks matching those on the murder victims. Cuthbert explains that he moved the crates after discovering one had been broken into, though only some seed pods appeared missing; the valuable Mbwun figurine was untouched. Whittlesey’s journal, the only documentation for the artifacts, has since gone missing. Examining the damaged crate, Frock observes that the claw marks appear on the inside of the boards, suggesting that something broke out rather than in. Cuthbert laughs dismissively.

Back in his office, an excited Frock tells Margo he believes the claw cast represents an evolutionary aberration that vindicates his Callisto Effect theory. He asks her to be his eyes and ears in the investigation.

Smithback takes a freight elevator to the osteological preparation laboratory to interview Jost Von Oster, an elderly Austrian who runs the lab where animal carcasses are reduced to bone. Von Oster enthusiastically shows him a maceration tank containing a decomposing zebra. When Smithback steers the conversation toward the Superstition exhibition, Von Oster calls Rickman “poison” and complains about her constant interference. He recounts how she cleared everyone from the exhibition hall yesterday in order to bring in the Mbwun figurine from the Kothoga tribe. He then adds to the gossip surrounding the figure, piquing Smithback’s interest.

On Wednesday afternoon, Margo finishes her write-up for Moriarty but learns that he has already gone to the exhibition. She calls Kawakita to ask about using the extrapolator but is annoyed when he puts her off. Determined to deliver her paper before the 5:00 pm curfew, she goes to the exhibition entrance but is turned away by a guard. Taking an alternate route to evade the guard, she finds a door unlocked and slips into a crawl space behind the exhibition walls.

As she navigates the gloomy, cave-like passages, she passes displays of tattooed Māori heads, fetishes, and burial scenes. In a dead-end gallery, she discovers a small figurine on a stone altar: Mbwun, the god of the Kothoga tribe, also known as “He Who Walks On All Fours” (134). She is horrified to see that the figurine has three talons on each forelimb, matching the murder weapon casts that Pendergast showed Frock. She hears a soft rustling nearby and smells a rank stench. Panicked, she flees.

After hiding and convincing herself that her fear was imaginary, she searches for an exit. A black shadow moves toward her through the darkness and she runs toward a pair of double doors and bursts through into the dimly lit main hall. As she leans against the doors, sobbing, she hears something clear its throat directly behind her.