In September 1987, Dr. Julian Whittlesey is leading an expedition through the
Amazon Basin in search of the lost Kothoga tribe, which worships a lizard god called Mbwun, hoping to prove that they still exist. Whittlesey disappears after finding the mutilated body of his partner. The following year, a dock worker in
Belém is brutally killed when a freighter arrives with a shipment of crates from the expedition. In 1995, in a fictionalized version of
New York City's
AMNH, two boys are found dead in a museum stairwell after getting lost after hours.
NYPD Lieutenant Vincent D'Agosta leads the investigation into the vast museum basement and subbasement, and discovers a claw embedded in one of the victim's brains. Meanwhile, museum leadership including
curator Winston Wright, deputy head Ian Cuthbert, and
PR director Lavinia Rickman attempt to keep the murders quiet in the lead-up to the grand opening of the new "Superstition" exhibition, which features wealthy benefactors. Rickman hires
NYT reporter Bill Smithback Jr. to cover the investigation, but edits his reports to be more palatable towards museum leadership. Employees spread the rumor of the "Museum Beast", a legendary monster that roams the subbasement. When a security guard is also killed,
FBI Special Agent
Aloysius Pendergast arrives to aid in the investigation.
Ethnopharmacology grad student Margo Green, her advisor, Dr. Whitney Frock, and Pendergast find that the claw from the autopsy match a Mbwun figurine sent back from Whittlesey's expedition. They also find letters that Whittlesey sent to a colleague, Montague, who later disappeared. Pendergast reveals that he is investigating the connection between the museum murders and the dock worker deaths, having traced the shipment of Whittlesey's crates between the incidents. However, the recorded contents of the crates reveal nothing but the figurine and dried plants. Margo and Smithback learn that Whittlesey claimed to have located the lost Kothoga tribe alive on a
tepui deep in the Brazilian jungle, as well as a unique plant its base that had protein-like qualities to it. The government bombarded the
tepui with
napalm in 1988, killing the remaining Kothoga. Margo and Frock realize that the plant Whittlesey discovered was in the crates, and run computerized analyses that find that the plants are rich in thalamoid hormones also found in small amounts in the human
hypothalamus gland, and the claw belonged to an unknown creature with simultaneous primate, reptile, and human genetics. They deduce that the creature is a predator of almost superhuman strength, speed, and intelligence, and is dependent on the now-extinct plant for nutrition; in its absence, it has been slaughtering humans and consuming their hypothalamus as a substitute. Under pressure from the museum leadership, the gala is allowed to begin, with security presence from the
NYPD and
FBI. Gala attendees find a body hidden above a display, causing a mass panic. At the same time, an NYPD officer is startled and opens fire on the security control room, destroying the central switching box and shutting down the museum's power. This causes the museum to enter an automatic security lockdown, trapping many people inside one section. Green, Frock, and Pendergast encounter the creature and shoot at it, only for the bullet to bounce off the creature's skull. D'Agosta leads Smithback and a group of gala attendees into the subbasement and tunnels beyond to seek escape, where the creature attacks, killing several people. The creature then finds the three directors hiding in an office, slaughtering Rickman and Wright, but mysteriously sparing Cuthbert. He is rescued by a SWAT team shortly before the entire team is killed. The group from the gala reaches the sewers through rising stormwater and finds the creature's lair, an underground chamber filled with corpses and trinkets, before escaping onto the street. In a final stand, Pendergast and Margo kill the creature by shooting it through the eye. 4 weeks later, the survivors convene, and Pendergast explains that the
Kothoga created the Mbwun by feeding a human the strange plant that Whittlesey discovered, which results in the subject transforming into the creature. When the
tepui was destroyed, the Mbwun followed the crates of plants to New York, living off the fibers for years and killing Montague when he came to investigate. The museum directors covered up evidence of Montague's disappearance and moved the crates into the heavily fortified vault; unable to access them, the Mbwun turned to murdering people and animals in the sewers and underground tunnels for their hypothalami. Meanwhile, Assistant Curator Gregory Kawakita performs his lab work and realizes that the creature was actually Whittlesey himself, after being forced to consume the plant by the Kothoga. Using the surviving plant samples, Kawakita develops a drug that turns users first into addicts, then into
Mbwun itself, and begins selling it on the street. He reflects that the Kothoga had no control over the Mbwun due to its unlimited access to the plant. In contrast, he is the only person alive capable of making the drug, and believes that he will now assert control over the creatures where the Kothoga failed. == Characters==