Jim and Artie go to Kansas and ride to the Buckley estate. The foreman, Fletcher, tells them to leave, informing the agents that they don't open the gates after sunset. Buckley's daughter Naomi finally come to investigate the commotion and tells Fletcher to let the agents in. However, as they enter the estate, more guards appear and Naomi explains that her father, Senator Seth Buckley, doesn't allow guns in the house. After disarming, the agents go to the study and wait for Buckley to arrive. When he does, they explain that Grant set them to bring Buckley to Washington since he ducked out of sight and there's now a national defense bill tied up in committee. Buckley tries to stall without success and refuses their offer as help... just as a man screams as shots are fired.
Jim runs out to investigate and finds the battered Fletcher on the ground, his lantern hanging from a nearby tree branch. As Fletcher tries to talk to Jim, someone puts out the lantern and runs off into the night. Buckley's three sons Aaron, Benjamin, and Caleb arrive with the guards and Jim tells them what happened. The agent prepares to go after the attacker but Aaron tells him that it's too dangerous to go. Jim ignores him and leaves, and Caleb complains that Aaron let Jim go off on his own. Aaron tells Benjamin to take some guards and stop Jim, and then orders Caleb to summon Dr. Von Liebig.
Jim follows the attacker's tracks and discovers that he broke into the estate grounds by ripping apart a section of metal bars with seemingly superhuman strength.
Aaron and Caleb have their men carry Fletcher into the house. Artie examines the foreman and discovers that his chest is crushed even though there are no signs of impact or beating. Buckley claims not to know what might be responsible, but Artie doesn't believe him. Fletcher recovers consciousness long enough to mutter something into the agent's ears and then passes out, and Artie tells the senator that Fletcher said a giant beast attacked him.
Jim follows the tracks and finds blood from one of Fletcher's gunshots. Benjamin and his three guards arrive and the Buckley brother tells Jim to go back to the house. When the agent refuses, the four men attack him and manage to subdue him. They roll Jim down a hill into the road where Dr. Von Liebig finds him as he drives his carriage to the estate. Von Liebig is glad to give Jim a lift back to the estate.
When Jim arrives at the estate, he and Artie exchange notes. They figure that the Buckleys have something to hide and won't let them check out the estate grounds. Von Liebig informs the agents that Fletcher died of his wounds and dismisses his description of the attacker as near-death delirium. Artie seems to recognize Von Liebig's name but the doctor says that he's no one important. Once he steps out of the room, Jim and Artie try to find Buckley and discover he's holed up with his sons in the study. Naomi tries to distract them by offering rooms and food, but is forced to admit that her family isn't seeing anyone. Von Liebig emerges from the study and leaves. When the agents knock, Aaron refuses to let them see his father, claiming that Buckley is still shook up over Fletcher's death.
The agents take Naomi up on her offer from earlier and ask why she isn't with the rest of the family. The woman explains that she's adopted and considered an outsider herself, and that her brothers are triplets and share a close bond. As they go upstairs, Artie notices the room directly above the study and asks about it, and Naomi informs him that it's the boys' nursery and off-limits. Once she's shown them their rooms, Naomi takes them to the dining room downstairs but Artie pleads fatigue and Jim leads her downstairs.
Once he's alone, Artie goes into the nursery and notices three cribs and a stained spot on the wall suggesting a fourth. Using a stethoscope, Artie listens in as the Buckleys argue about the killer. Aaron warns that the murderer is after them and would have gotten to them if he hadn't encountered and killed Fletcher. Caleb comments on how the killer would have only killed "every known Buckley." Buckley himself doesn't believe what is happening, but refuses to give Aaron permission to deal with the killer. Artie bores a hole in the wall and inserts a periscope, and watches as Aaron warns that there will be a bloodbath if they don't act soon. The senator goes to bed and Caleb worries that someone may stumble across the birth records at the nearby church. Benjamin claims he isn't afraid of the killer and Aaron tells him to go to the church and burn the records. While Artie concentrates, he fails to hear Buckley, who comes up behind him and clubs him unconscious with his cane.
After they eat, Naomi tries to take food to Artie. Jim tries to stall her but Artie staggers out of the nursery, bleeding from his head wound. Suffering from partial amnesia due to the concussion, Artie finally remembers that Benjamin was going to get some records.
At the church, Benjamin looks for the windows, unaware that a huge man is watching him from the window. The observer smashes in and Benjamin looks at him in horror, uttering the name, "Dimas."
Naomi and the agents hear the church bells ring and Artie finally remembers that Benjamin was going to the church. Jim runs out of the manor past the guards and goes to the church, and finds Benjamin's crushed body on the floor. The minister, Reverend Hastings, comes in and protests the desecration of the church bells until he realizes that there's been a murder. He glances out the window and tells Jim that he saw a giant ape run into the woods.
Back at the manor, Jim checks on Buckley and confirms that he's sleeping. While the two agents try to work out their next step, Artie finds a research book on the shelves about the great apes. The author is Von Liebig and Artie finally remembers that Von Liebig is a famous anthropologist. He tells Jim to stand guard over Buckley while he talks to the doctor. Posing as a fellow anthropologist, Artie tries to get Von Liebig to talk about his work with gorillas. Von Liebig claims he is no longer interested in the field of anthropology and then yells at someone moving around in the back. He then tells Artie that he has to attend Benjamin's funeral and leaves. Artie goes first and hides in the hallway closet. Von Liebig fails to see him when he gets his coat and leaves, and Artie slips into the back.
Family and friends gather at the church for the funeral service and Jim suggests that Hastings' daughter, Priscilla, take Naomi inside while they go to the graveyard.
Artie discovers that Von Liebig has a well-stocked laboratory in his back room, including a cell. He activates a hidden switch and the giant from the church, Dimas, steps out of a hidden passage. Artie fails to bluff him and the giant attacks him, forcing him to duck into the cave and lock himself in. Dimas breaks off a metal bar in the lock, trapping Artie, and then leaves. However, Artie quickly burns himself free using thermite. He goes to the funeral and afterward, tells Jim what happened. They figure that Dimas killed Fletcher and Benjamin. Before they can investigate further, Priscilla tells them that Naomi went into the forest with a giant monkey. The agents realize that someone else is involved and Artie stays to watch Buckley while Jim goes into the forest. He follows the tracks and finds one of Naomi's gloves caught under a rock. He pulls it up, revealing a hidden tunnel below. Entering it, Jim goes to the far end and finds a large cavern with lamps and cells. Naomi is locked into one cage and Jim starts to free her. However, Dimas arrives and releases his ally: a gorilla, Johann. Jim tries to fight back but is soon overpowered.
That night, Aaron goes into town and rouses the townspeople into going to the estate and finding the murderer. As the mob approaches, Artie and Buckley watch from the window. Artie tells the senator that he has to tell his truth and stop his sons, but Buckley refuses and figures that Jim may be dead by now.
Dimas imprisons Jim in the hanging cage with Naomi and puts Johann in his cage. He then tell the prisoners that he planned to use Naomi as bait to bring in the remaining three Buckleys and kill them. He ignores Jim's question about why he wants revenge, releases Johann to stand guard, and then leaves.
While they wait, Artie reveals to Buckley that he knows about Dimas, having found his birth record in the family Bible. Buckley admits that his wife had quadruplets, not triplets. Dimas supposedly died and was buried in the cemetery, and Von Liebig comes in, aiming a shotgun at Artie. He explains that he faked Dimas' death and forged the death certificate. Artie tells him that Johann is out of control and the doctor reluctantly hands over the shotgun. He explains that he tried to raise Johann as a human in the hopes that he could be taught to be human, but the training never took and he was unable to control the beast.
Jim uses a lockpick to get the cage door open, pours flammable powder on Naomi's cloak, and then throws it at the gorilla to distract it while he and Naomi escape.
Buckley explains that he had Von Liebig fake Dimas' death because the boy was different than his brothers, and hoped that the anthropologist could help him. Artie figures that Buckley also didn't want a liability as a son as well. Buckley explains that Dimas eventually discovered his heritage and wants revenge, and plans to use his "friend" Johann to kill the family and inherit everything.
Dimas breaks into the manor, entering through the second-story window into the nursery. He finds his old teddy bear and looks at it affectionately, winding it up. However, when Caleb hears the music and comes in, Dimas kills him.
Artie and the others hear Dimas upstairs as Jim and Naomi arrive at the house. Jim goes up to confront Dimas and after a brief struggle, knocks Dimas over the railing. Johann bursts in through the window and Artie gets off a shot before the gorilla begins to crush him. However, the gorilla sees the dying Dimas and goes to him, and then runs outside. Aaron and the mob are outside and open fire on the gorilla, killing it. Jim arrives and sees what they've done, and turns and goes back to the manor in disgust. Inside, Buckley is holding his dead son and wondering what he caused by his actions.