The Monstrumologist begins when Erasmus Gray visits Dr. Warthrop's house in the middle of the night. He reveals the body of a young woman and a male anthropophagus, of a species called Anthropophagi. The girl has "half her face" missing, and her throat is torn out. Dr. Warthrop dissects both the girl and the monster, and finds an Anthropophagus fetus in the dead girl's womb. Warthrop explains to Will Henry, who is taking notes on the procedure, that Anthropophagi require a host to grow their young in. They poison the baby and continue examining the bodies. Dr. Warthrop theorizes that because Anthropophagi are indigenous to Africa and have never been seen in the Americas before, there could only be one or two more in the area at most. The next night, Warthrop, Gray, and Will Henry go to the cemetery to return the girl's body to her grave.
While Gray and Will Henry re-dig the grave, Warthrop tries to discover how the Anthropophagus accessed her body in the first place. Suddenly, an Anthropophagus bursts up from under the dirt in the hole Will Henry and Gray were digging. It seizes Gray by the legs and starts dragging him down. Will Henry grabs Gray's wrists and tries to pull him back, but starts being pulled in himself. He tries to release Gray, but Gray hangs on to him and refuses to let go. Dr. Warthrop shoots Gray and pulls Will Henry back, while Gray's body is devoured.
Warthrop and Will Henry run for their lives as more Anthropophagi pour out of Eliza's grave. They jump on the back of Gray's horse and ride off, narrowly evading the approaching swarm. Back at Warthrop's house, he and Will Henry begin searching through old newspapers for suspicious deaths over the past thirty years. Warthrop marks possible Anthropophagi attacks on a map, and estimates that there are around twenty five or thirty Anthropophagi in the area. The map also shows that the Anthropophagi have been steadily making their way from the coast to New Jerusalem (where Warthrop lives) over the last couple of decades.
Warthrop then composes a letter to John Kearns asking him to come and use his "inestimable services" to help with the infestation. Then, in some of Warthrop's father's old journals, he finds a reference to a place on Motley Hill, in Dedham, that might be connected to the Anthropophagi's arrival. Will Henry finds a key among the old papers as well, and puts it in his pocket to show to Warthrop later. Will Henry also realizes that he has lost his hat, the last object in his possession from his home before his parents died. Warthrop and Will Henry leave that night for Dedham, where they find the Motley Hill Sanatorium. A woman named Mrs. Bratton answers and refuses to let them in, until Warthrop mentions he's the son of Dr. Alistair Warthrop.
Dr. Starr lets them in and reveals that Warthrop's father had a keen interest in one of Starr's patients, Captain Varner. Warthrop asks to see Varner. Starr stalls for several minutes, until Warthrop bribes him. Mrs. Bratton leads Warthrop and Will Henry upstairs. As they pass doors, they hear patients crying and calling out. They're also surrounded by the smell of "unwashed flesh, old urine, and human feces." They go into the last door. Under the window "clustered the bodies of desiccated flies. Above them, a congregation of their extant cousins buzzed about and crawled upon glass. My eyes began to water, for the smell of bleach was overwhelming, and I deduced the reason for delaying the doctor downstairs: Mrs. Bratton had needed time to scour and disinfect before our introduction to Captain Varner."
Warthrop asks Mrs. Bratton to leave, which she initially refuses. When Warthrop insists, she goes back into the hall. Warthrop then tries to get Varner to speak, but without any success at first. It's not until the name Warthrop is mentioned that he really responds. Varner tells them that he was hired to sail to Benin (in Africa) about twenty five years previously, to return with a "cargo of particular interest." He was paid a large amount of money to pick up three Anthropophagi, an adult of each gender and a cub, and sail them back to the New World. Varner and his crew were instructed to lock up the Anthropophagi in the bottom of the ship and throw down a cow, goat or chimpanzee to them every couple of weeks. However, the Anthropophagi refused to eat them.
The crew makes a sport of tormenting them, particularly the first mate. Drunk one day, the first mate takes a piece of cow meat and swings down a rope in the top of the Anthropophagi's holding area below deck to give it to them. The female Anthropophagus grabs him and eats him. Captain Varner posts guards to ensure nothing like it happens again, but one day a huge storm hit the ship and the crew can see almost nothing. The Anthropophagi climb through the porthole and onto the deck, where they proceed to slaughter the crew When only Varner is left, he runs for the lifeboats, stabbing one of the female Anthropophagi's eyes on the way out. When he returns home, he is sent to the sanatorium.
Warthrop pulls back Varner's bedding to reveal that Varner's flesh is being eaten away by maggots, thus the flies. Warthrop diagnoses that whatever infection Varner has already spread to his bones: Varner will live no more than a day. Warthrop spends the night at Varner's bedside until Varner dies the next morning. It is understood that his death is the direct result of neglect from Starr and Bratton. He then confronts Mrs. Bratton, informing her that he is going to inform the police that they are mistreating their patients.
Early the next morning, Constable Morgan knocks on the door asking for Warthrop. He tells Warthrop that something has happened "totally outside the range of my experience." On the carriage ride to the crime scene, Warthrop questions Morgan. Warthrop discovers that "the crime" was reported "shortly after dawn" Only one person survived (Malachi). Morgan explains he called on Warthrop because "no human being is capable of so foul a crime."
They arrive at the town rectory. (224) Five have been found dead; the rector, his wife, and three of their children. Blood and carnage are everywhere. Warthrop estimates it to be the work of at least eight to ten Anthropophagi. Morgan observes that the odds of these monsters appearing in the town by chance are very small.
Warthrop, Morgan and Will Henry go to the sanctuary, where Malachi is waiting. He is visibly disturbed and does not respond much to anyone except Will Henry, after discovering Will Henry's family is also dead, and that Will Henry ran from the scene. Malachi explains what happened before he ran. The Anthorpophagi came through the windows of the house while they slept. One of his sisters came to hide in his room with him. They listened to their families screams and the sounds of the monsters tearing them and the house apart. Malachi breaks open his bedroom window so he and his sister can escape. The Anthropophagi hear the sound of breaking glass, and his sister faints. Malachi tries to drag his sister through the window, but an Anthropophagi comes in and grabs her. Malachi jumped through the window and rode to Morgan.
Warthrop and Will Henry return home. Morgan arrives shortly. He brings Malachi and his assistant with him. (253) He tells the doctor he has been at the cemetery, and found Will Henry's hat. Morgan deduces that Warthrop knew the Anthropophagi were there, which Warthrop confirms. Malachi grabs Morgan's assistant's gun and jumps on top of Warthrop. Malachi blames Warthrop for his family's death. Will Henry convinces him to let go. Warthrop takes Morgan and his assistant to see the Anthropophagi body that Gray brought to him, which is still in his basement.
Kearns arrives and converses with Warthrop, Kearns behaving very warmly, Warthrop just the opposite. Morgan appears, and Kearns introduces himself as Richard Corey. (266) The three men look at Warthrop's map where he plotted the various Anthropophagi attacks. Kearns suggests Warthrop's father paid to have the Anthropophagi shipped over, and Warthrop slaps him. As the night goes on, Kearns continues to tease Warthrop about his relationship with his father.
The next day, Warthrop, Kearns, Will Henry, Malachi, Morgan, and six of Morgan's men meet to prepare a trap for the Anthropophagi. Kearns is in a notably good mood. Kearns directs the men in setting up the 'slaughter ring in the cemetery.' Morgan becomes steadily more and more disgusted with Kearns. When Kearns states that "there is no morality save the morality of the moment," Morgan retorts "I begin to see why you delight in hunting them. You've so much in common."
Once the men have finished setting up the 'slaughter ring,' Kearns pulls out the bait he brought with them: a woman's motionless body, "reposed as a corpse." Morgan protests that using the woman's body is immoral. Kearns justifies that "it's a woman of the streets, Morgan." Warthrop, though reluctant, concedes the necessity of bait. As evening comes, everyone gets into position in the trees around the slaughter ring, guns and grenades in hand.
Kearns has the woman's body chained in the middle of the slaughter ring. Suddenly she starts kicking and screaming, and the other men realize she is alive. "With a single fluid motion of his arm Kearns reached across the thrashing captive's torso and opened up her abdomen with the razor-sharp blade." Kearns runs back into the trees where the rest are waiting. Morgan curses Kearns, but there is no time to help the woman as the Anthropophagi start to come up from underground. Kearns shoot the first one to injure, not to kill, in the hopes that its suffering will draw out the rest.
Warthrop runs out to the screaming woman, ignoring the injured Anthropophagi, carries her under the trees and starts binding up her wounds. At the sound of more Anthropophagi approaching, Warthrop returns to the trees to shoot, leaving Will Henry and Malachi to tend to the woman. As the adults in the trees deal with the largest faction of Anthropophagi, a juvenile comes upon Will Henry and Malachi. Will Henry pulls out Warthrop's revolver, but the Anthropophagi smacks it out of his hand and grabs him. (328) Will Henry pulls out a knife from his belt. He stabs each of the Anthropophagi's eyes, and then its brain, killing it. The battle ends shortly. Warthrop has Morgan's assistant take the wounded woman to a doctor.
Upon examining the Anthropophagi bodies, they cannot find the leader, who they expect to recognize by her missing eye (as she was stabbed by Varner earlier on). Surmising she is still underground with her young, they decide they must go into the Anthropophagi warren and look for her in order to ensure that all of the monsters are killed. Kearns suggests that the "front door" of the Anthropophagi's underground home must be in the Warthrop mausoleum, in keeping with his theory that Warthrop's father had them brought over in the first place. Inside it, Kearns finds a hidden clock. Warthrop, finally accepting that Kearn's theory must be true, find a hidden door in the clock.
They find a locked trapdoor. Will Henry remembers the key he found earlier (still in his pocket), and tries it out: The trapdoor opens. They lower themselves into what appears to be the main pit where the Anthropophagi live and eat; the floor is covered in bones. They search the tunnels until Kearns and Warthrop discover a small tunnel that only Will Henry can get through. Will Henry crawls through it for a long time until he falls into another pit. He finds another juvenile Anthropophagi sleeping. It appears to be in considerable pain. Wanting to put it out of its misery, Will Henry comes up close to shoot it. The Anthropophagi jerks awake and grabs Will Henry. In the struggle, Will Henry's arm is bitten. Will Henry eventually manages to kill it with a large stone.
Will Henry wanders around in the tunnels until Kearns finds him. Will Henry explains what happened to him, and Kearns then instructs him to take the 'bandage' (Will's shirt, which he has wrapped around his arm) off of his bite wound, because "we don't want to risk an infection." Kearns then instructs Will Henry to follow the path Kearns has marked, which Kearns says will take Will Henry back to Warthrop. Will Henry follows Kearns instructions. After a little while, Will Henry thinks he hears something following him. Will Henry continues, until he smells Anthropophagi ahead and stops. Kearns appears behind him and Will Henry realizes Kearns has tricked him into going straight into the remaining Anthropophagi's lair.
"'I had hoped to avoid it,' was his cryptic response. 'The smell of blood should have drawn her out; I'm at a loss, frankly, why she didn't come... I am so sorry, Mr. Henry, but there really is no choice. It is the morality of the moment.' And with those parting words John Kearns shoved me as hard as he could." Two juvenile Anthropophagi leap towards Will Henry; he shoots one, and above him, Kearns shoots the second. As a third sails towards Will Henry, Kearns shoots the last monster.
Warthrop comes running over to Will Henry, who explains what Kearns did. When Kearns tries to justify his actions, Warthrop threatens to shoot him. Suddenly, the floor bursts from under them as the leader pushes up and grabs Malachi. Malachi tells Will Henry to grab a grenade from Kearn's bag; Will Henry tosses it at Malachi and the matriarch, and they are both buried in dirt, mud and rock. The matriarch rises out of the dust, leaping towards Will Henry, who knows he only has one shot left in his gun. He aims for her groin and shoots: she dies.
In the final chapter, we learn that Warthrop, Kearns and Will Henry visit Starr afterward; Kearns kills him, then disappears. Finally, Warthrop gives Will Henry a new hat: after holding his old hat in one hand and his new one in the other, Will Henry tosses his old hat in the fire.
Filled with suspense and gore, The Monstrumologist "might just be the best horror novel of the year." "The industrial-era setting is populated with leering, Dickensian characters."
The review in Publishers Weekly said, "Yancey's elegant depiction of an America plagued with monsters, human and otherwise, spares no grisly detail. ... Horror lovers will be rapt." The reviewer in the School Library Journal wrote "Though the pace sometimes falters beneath the weight of Will's verbose observations, the author folds surprising depth and twists into the plot and cast alike, crafts icky bits that can be regarded as comically over-the-top (or not), and all in all dishes up an escapade fully 'capable,' as Will puts it, 'of fulfilling our curious and baffling need for a marauding horror of malicious intent'".