In Kansas, Jim and Artie ride up to the Buckley estate and the foreman, Fletcher, warns them that they don’t open up the gates at night. When the agents identify themselves, Buckley’s daughter Naomi comes out and tells Fletcher to let Jim and Artie in. As she leads them in, Naomi tells them that her father doesn’t allow guns in the house. The agents remove their holsters and wait in the study while Naomi goes to get her father. They wonder briefly why the security is so intense, but Senator Seth Buckley comes in and interrupts them. The agents explains that President Grant hasn’t heard from him in sometime and that he needs Buckley to return to Washington because he has a national defense bill tied up in committee. Buckley tries to put them off but the agents offer their help. The senator insists that he’s not in any difficulty... just as a man screams outside and gunshots ring out.
Jim runs to investigate and finds the badly injured Fletcher on the ground in the light of his lantern, hanging from a branch. The foreman tries to mutter out a few words, but someone puts out the lantern and slips away in the darkness. Buckley’s sons Aaron, Benjamin, and Caleb come running up with their men and Jim explains what happened. He tells the Buckleys to get a doctor for Fletcher while he tracks down the attacker. Aaron tells him to stay put, insisting that it’s dangerous, but Jim leaves them. Once he’s gone, Caleb notes that Aaron shouldn’t have let Jim go, but Aaron tells his brother to summon Dr. Von Liebig. Benjamin insists that he can handle Jim and Aaron agrees to let him take some men and stop Jim from exploring.
The agent follows the attacker’s tracks to the wall and discovers a section where someone with superhuman strength pulled apart the bars.
The Buckleys get Fletcher into the house and Artie confirms that his body is literally crushed although there are no signs of a beating or a fall. Senator Buckley insists that he doesn’t know what might have caused Fletcher’s injuries, but Artie demands to know why the high security. Before Buckley can answer, Fletcher recovers consciousness and mutters something into Artie’s ear. Fletcher passes out and Artie tells the senator that the foreman said he was attacked by an enormous beast of some kind.
Jim continues following the tracks across the manor grounds and finds some blood. Benjamin and his men catch up to him and tell Jim that he’s wanted at the house. When Jim refuses, a fight breaks out and Benjamin and his three men soon subdue him and roll him down a hill into the road. As Jim gets up, a wagon pulls up and Dr. Von Liebig checks on Jim. The agent tells him that they need Von Liebig at the manor and they offer him a ride back.
Later, Artie tells Jim what Fletcher told him and Jim tells him what he saw at the gate. He warns Artie that the Buckleys have something to hide and that they won’t let them search the grounds. Von Liebig dismisses Fletcher’s claim as delirium and informs the agents that the foreman is dead. Artie finds Von Liebig’s name familiar but the doctor says that there’s no reason that he would know him. Once Von Liebig leaves, Jim and Artie look for Buckley. They find Naomi, who tries to offer them a meal and rooms, but they realize she’s trying to keep them from her father. She admits that the rest of the family is in caucus and they brush past her.
Von Liebig comes out of the study and leaves as the agents knock, and Aaron asks what they want. He refuses to let them see his father until he’s had a chance to recover from the shock of Fletcher’s death. Realizing they need another way in, the agents turn to Naomi and accept her hospitality. Artie asks why she isn’t with the others and Naomi explains that she’s adopted and the three brothers are triplets. As they go up to their rooms, Artie notices a room directly above the study and asks where it goes. She says that it’s the boys’ nursery and that it’s strictly off-limits. Naomi then offers them a hot meal, but Artie begs off and Jim quickly leads her back downstairs.
Once he’s alone, Artie goes to check out the nursery. Looking around, he finds three cribs but a spot on the wall indicating the former presence of a fourth. Artie then uses a stethoscope to listen in on the conversation in the study. Aaron insists that if not for Fletcher, the killer would have come after them and wiped out every Buckley. Caleb corrects him, commenting on “every known Buckley.” Senator Buckley can’t believe what’s happening and Aaron asks for his permission to stop it. Meanwhile, Artie drills a hole in the floor and watches them, as Aaron insists that there’ll be a bloodbath if they don’t act soon and that there’s no reasoning with the killer. Buckley refuses to sanction their actions and goes to bed, and Benjamin says that he’s not afraid of their opponent. Caleb worries that there’ll be an investigation into the church records of the Buckley family. Aaron promises that they’ll destroy them and tells Benjamin to go there and burn them. Artie, concentrating on the conversation, doesn’t hear Senator Buckley come up behind him and club him unconscious.
When Naomi tries to take some food up to Artie, Jim tries to stall her. The injured Artie staggers out of the nursery but has trouble remembering what happened. He finally recalls that Benjamin went out to get some records.
Benjamin goes to the church alone and looks for the records, while a gigantic figure watches him from the window. He smashes through and Benjamin calls out a name, “Dimas.”
At the house, Naomi hears the church bells ringing at midnight, and Artie remembers that they sent Benjamin to the church. Jim runs out past Aaron and his men and goes to the church. He finds Benjamin, his body crushed. Reverend Hastings comes in and protests the desecration of the church bells. Jim explains that Benjamin is dead and Hastings sees what appears to be a giant ape running into the nearby woods.
Jim returns to the manor and confirms that the senator is sleeping. He and Artie both agree that they can’t leave without resolving the situation, and figure that Senator Buckley will take his family secret to the grave. Artie goes through the library and finds a research book confirming that Von Liebig is an anthropologist specializing in the great apes. He goes to talk to Von Liebig and explains that he’ll be posing as an anthropologist.
Artie presents his fake credentials to Von Liebig, who insists that he’s abandoned his research into great apes and simply works as a country doctor. Something moves in the back and Von Liebig yells in German and then tells Artie that he has to attend a funeral. Artie quickly leaves and Von Liebig goes to get his coat... unaware that Artie is hiding in the closet. Once the doctor leaves, Artie slips out of the closet and goes to check out the back.
At the church, Jim suggests that Naomi go inside with Hastings' daughter, Priscilla, while he and the others go to the grave.
Artie searches Von Liebig’s office and finds a cage. When he examines a skeleton, he sets off a hidden switch and opens a panel. Dimas, the giant from the church, comes out and Artie tries to bluff his way out. Dimas pulls a chain out of the wall and swings at Artie, who locks himself in the cell. The giant starts to pull the bars apart, but the church bells ring and Dimas pushes the bars back and then jams the lock. Once his attacker leaves, Artie uses thermite to burn through the bars.
Artie goes to the cemetery and attends Benjamin’s funeral, and tells Jim what happened. He figures that Dimas could be the killer. Priscilla comes over and tells Jim that Naomi went into the woods with what she describes as a giant monkey. Artie admits that it doesn’t sound like Dimas and he stays to watch the senator while Jim follows the trail. The agent finds one of Naomi’s gloves beneath a slab and pulls it aside, revealing a hidden complex below. Jim finds Naomi in a cage and starts to free her, but Dimas releases the gorilla, Johann, into the tunnels. As the giant laughs in triumph, Johann finally pins down Jim and starts to crush him.
Aaron mobilizes the townspeople into a mob to kill Johann and avenge his brother’s death. Artie watches from the manor and tells Buckley that he has to stop his sons and tell the truth. The senator points out that they haven’t heard from Jim and figures that he may be dead as well.
Dimas puts Jim in the cage with Naomi and returns Johann to his cage. He explains that he hoped to lure the Buckleys to him using Naomi as bait and refuses to explain why. Dimas then release Johann to stand guard and leaves.
In the parlor, Artie asks the senator who Dimas Buckley is. The agent has gone through the family Bible and discovered that Senator Buckley and his wife had four children: Benjamin, Aaron, Caleb, and Dimas. The senator explains that Dimas died and was buried in the cemetery. When Artie wonders who signed the death certificate, Von Liebig comes in with a shotgun and says that he did. The agent points out that the gorilla has been seen in broad daylight and Von Liebig hands the shotgun to him. The doctor says that it’s too late and, ignoring the senator’s warning to remain silent, explains that he raised Johann and trained it to prove his thesis that if it were raised in a human environment, it could attain the same degree of development. However, he admits that he is unable to control Johann.
Jim picks the lock on the cage and then tells Naomi to block Johann’s line of sight. He then pours some powder onto Naomi’s cloak, sets it fire, and ducks out of the cage with Naomi while the gorilla is distracted.
Senator Buckley admits that he gave Dimas to Von Liebig because the boy was different from his brothers. He hoped that the doctor could help him, but Artie points out that a son like that would be a liability to a politician. The senator says that Dimas discovered his heritage and plans to use Johann to wipe out the Buckleys and inherit everything.
While Jim and Naomi run to the manor, Dimas scales the walls of the house and enters the nursery through a window. He finds his old wind-up teddy bear and holds it affectionately. Caleb hears the music and comes to investigate, and Dimas kills him.
Artie and the others hear the noise just as Jim and Naomi come in. Jim warns them that Johann is outside and figures Dimas is upstairs. He goes to investigate and Dimas attacks him. After a brief struggle, Jim knocks the fourth Buckley unconscious. Johann storms in through the window and Artie manages to get off a single shot before the gorilla attacks him. However, Johann goes to his friend for a moment, and then runs outside. Aaron and the mob enter the grounds and fire at the gorilla, killing him. Jim arrives and looks at Aaron and the others in disgust and then goes back inside. Senator Buckley is holding his long-lost son, wondering what he did to him.