Plot
In September 1987, Dr. Julian Whittlesey is leading an expedition through the Amazon Basin, in the Brazilian rainforest, in search of the lost Kothoga tribe. He hopes to prove that they still do exist and in the process learn more about their culture, including their lizard god Mbwun ("He Who Walks On All Fours"), supposedly the son of Satan. However, Whittlesey disappears after finding the mutilated body of his partner, Crocker, and realizes that a creature in the bush is stalking him. 12 months later, a dock worker in Belém named Ven is suddenly and brutally killed when a freighter arrives with a shipment of crates from Whittlesey's expedition.
Seven years later, in a fictionalized version of New York City's American Museum of Natural History, two young boys are found dead in a museum stairwell after getting lost after hours. NYPD Lieutenant Vincent D'Agosta leads the subsequent investigation, keeping the museum under tight lockdown and its staff placed under curfew for fear the murderer is still hiding somewhere in the museum or the many catacombs that run beneath it. The three prominent leadership figures of the facility—curator Winston Wright, deputy head Ian Cuthbert, and public relations director Lavinia Rickman—attempt to keep the murders under wraps in the lead-up to the grand gala opening of the new "Superstition" exhibition, led by George Moriarty, which feature many wealthy benefactors. Rickman hires The New York Times reporter Bill Smithback, Jr. to cover the murder investigation, but repeatedly edits his reports so they will appear more palatable towards the museum and its leadership. Employees spread the rumor of the "Museum Beast", a legendary monster that has allegedly been roaming the tunnels under the museum for years. D'Agosta is shocked during the autopsy of the two boys when he discovers a claw buried in one of the boy's brains.
When a security guard suffers a similarly brutal death, FBI Special Agent Aloysius Pendergast arrives to aid in the investigation, while the search spreads to the tunnels under the museum. Ethnopharmacology grad student Margo Green and her mentor, Dr. Whitney Frock, discover with Agent Pendergast that the claw from the autopsy matches the claws on a Mbwun figurine sent back to the museum in a crate from Whittlesey's expedition. Margo steals letters from a secure vault that Whittlesey had sent to a museum colleague named H.C. Montague who later disappeared, detailing the mysterious events from the 1987 expedition. Pendergast reveals that his interest in the case is due to the similarity of the museum murders to the murder of the Brazilian dock worker in 1988, as well as several other dock workers in Louisiana later that year, and that he traced the shipment of Whittlesey's crates as a connection between the incidents. However, other than the Mbwun figurine, the crates were empty but for a quantity of local plant material seemingly used as packaging. Pendergast hints at the possibility that the murders might not have been committed by a human, and that the Mbwun monster may somehow have been sent to the museum by Whittlesey's shipment.
Margo and Smithback discover a colleague of Whittlesey's, to whom one of the letters was addressed, still working at the museum, and learn from him 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 at the base of the tepui that had protein-like qualities to it. Intending to mine the entire area, the government bombarded the tepui with napalm in 1988, killing all of the remaining Kothoga and destroying the plant. Green and Frock realize that the plant Whittlesey discovered was the same plant that had been sent back in the crates and run computerized analyses on them and the claw; the results reveal that the plants are rich in thalamoid hormones, which can also be found in small amounts in the human hypothalamus gland in the brain, and that the claw belonged to an unknown creature with simultaneous primate, reptile, and human genetics. Green and Frock deduce that the creature is a predator of formidable strength, speed, and intelligence, and is dependent on the plant fibers for nutrition; in its absence, it has been slaughtering humans and consuming their hypothalamus as a poor substitute.
Under pressure from the museum leadership, the gala is allowed to begin, with combined security presence from the NYPD and FBI. In the middle of the exhibition, gala attendees notice a body hidden in the ceiling above a display, causing a mass panic. At the same time, an NYPD officer panics at a noise and opens fire on the security control room, completely destroying the central switching box and shutting down the museum's power. This causes the museum to go into an automatic security lockdown, trapping a large group of people at the gala inside one section of the museum, including D'Agosta, Smithback, Cuthbert, Wright, and Rickman. Green, Frock, and Pendergast encounter the creature, and Pendergast shoots at it, only for the bullet to bounce off the creature's skull. Contacting the trapped group led by D'Agosta, Pendergast warns him of the creature, and advises them to escape down through the museum tunnels. As they attempt to do so, the creature attacks, killing several people, and forcing the rest down into the tunnels. The creature then enters the office where the three directors are hiding, and slaughters Rickman and Wright while leaving Cuthbert alone. Cuthbert is rescued by a SWAT team shortly before they move in to attack the creature, only for the entire team to be killed.
The group from the gala reaches the sewers through rising stormwater, losing another police officer to an attack by the creature, before arriving in the creature's lair, an underground chamber filled with corpses and trinkets. Finally, they are able to escape onto the street via a manhole. Pendergast and Margo make a final stand against the creature, with Pendergast attempting to shoot out the creature's legs to stop its charge; at the last second, Green shouts for Pendergast to shoot through the creature's eye, causing the bullet to pass into the brain, killing the creature.
Four weeks later, Green, Frock, Kawakita, Pendergast, D'Agosta, and Smithback convene, and Pendergast explains that the Kothoga created the Mbwun by feeding a human the strange plant that Whittlesey had discovered and shipped back to the museum, which results in the subject transforming into the creature known as Mbwun. When the tepui was destroyed, the Mbwun followed the last surviving samples of the plant in Whittlsey's crates all the way to New York. It lived off the plants for years, and killed Montague when he came down to investigate the crates. 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 hypothalamus, before the murders of the boys and the security guard. Pendergast also reveals that among the bodies that were found in the lair were the remains of George Moriarty, and a pendant that Whittlesey had always worn on him.
In his lab, assistant curator Gregory Kawakita realizes that the creature itself was in fact Whittlesey after being forced to consume the plant by the Kothoga. Using the surviving samples of the plant, Kawakita is developing a drug that would turn the 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 because of unlimited access to the plant, whereas 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.
