Lair of the White Worm

The central character of the book is Adam Salton, an Australian (living there at the outset) who in 1860 is contacted by his elderly great-uncle, Richard Salton, a landed gentleman of Lesser Hill, Derbyshire, England, who has no other family and wants to establish a relationship with the only other living member of the Salton family. Although Adam has already made his own fortune in Australia, he enthusiastically agrees to meet his uncle. On his arrival by ship at Southampton, the two men quickly become good friends. His great-uncle then reveals that he wishes to make Adam the heir to his estate, Lesser Hill. Adam travels there and quickly finds himself at the center of mysterious events, with Sir Nathaniel de Salis, a friend of Richard Salton's, as his guide.
Edgar Caswall, the new heir to a neighbouring estate, Castra Regis or Royal Camp, is in the process of making a mesmeric assault on a local girl, Lilla Watford. Meanwhile, Arabella March, of Diana's Grove, is running a game of her own, perhaps angling to become Mrs. Edgar Caswall. Caswall is a slightly pathological eccentric and has inherited Franz Mesmer's chest, which he keeps in the Castra Regis Tower. Caswall seeks to make use of mesmerism, a precursor to hypnotism, and, obsessed with Lilla, attempts to break her using mesmeric powers. However, with the help of Lilla's half-Burmese cousin, Mimi Watford, he is thwarted time and again.
Caswall has a giant kite built in the shape of a hawk to scare away pigeons that have attacked his fields and destroyed his crops. For lack of anything better to do, he obsessively watches the kite and begins to believe that it has a mind of its own and that he himself is a god.
Adam Salton finds black snakes on his great uncle's property and buys a mongoose to hunt them down. He then discovers a child who has been bitten on the neck and who almost dies as a result. Adam learns that another child has already been killed by a snake bite and that animals have also been killed mysteriously throughout the county.
Caswall's Black African servant, Oolanga, a man obsessed with death and torture, prowls around the Castra Regis estate, enjoying the carnage left by the White Worm. Adam's mongoose attacks Arabella, who shoots it to death. Adam buys more mongooses and keeps them locked in trunks when not using them to hunt. Arabella tears another mongoose apart with her hands. Oolanga takes a liking to Arabella, perhaps sensing something violent in her, and makes advances. Arabella scorns Oolanga and is deeply insulted that he would dare to approach her. In an attempt to win her over, Oolanga steals one of Adam's trunks (which he believes is filled with treasure, but is actually just another mongoose), and Adam follows Oolanga. Arabella lures Oolanga to a deep well in her house, then in rage and disgust murders him by dragging him down into the deep pit tunneled through a bed of white china clay. Adam witnesses the murder, but has no evidence of it apart from his own word. Arabella writes him a letter the next day, with the previous night's events twisted, claiming her complete innocence. Adam and Sir Nathaniel begin to suspect that Arabella is guilty of other crimes and that she wants to murder Mimi Watford. To further protect her, Adam marries Mimi secretly.
Adam and Sir Nathaniel then plot to stop Arabella by whatever means necessary. Sir Nathaniel is a Van Helsing-type character who wants to hunt down Arabella, who he believes, with increasing conviction, is the supernatural human embodiment of the White Worm of legend. She exploits the White Worm and has acquired characteristics similar to those of the creature. This is only his theory, however, which is not corroborated by any evidence or facts in the story itself.
The White Worm is a large snake-like creature dwelling deep under Arabella's house at Diana's Grove. It has green glowing eyes and feeds on whatever living creatures it can find to eat. Sir Nathaniel believes the Worm is descended from dragons, who traded their physical power for cunning. The Worm ascends from its pit and seeks to attack Adam and Mimi Watford in the forest of Diana's Grove.
The White Worm pursues Adam, Sir Nathaniel, and Mimi in a long chase as they seek to outpace it in a horse-drawn coach. A boat awaits them on the shore. This allows them to chase down the ship and to board it. The White Worm enters the water and stalks them but they are able to elude it and escape. The White Worm is described as a "whale" by observers because of its large size.
Adam is able to foil Arabella's multiple attempts to murder Mimi. In financial straits, Arabella offers to sell Diana's Grove, which Adam buys with the aim of destroying the White Worm. He plans to fill the pit with sand and set dynamite to kill the Worm while it is underground.
Caswall's last visit to Lilla ends in her death after his mesmeric attack leaves her totally drained and exhausted. Arabella joins Caswall as Mimi enters the room but is unable to save Mimi.
In the final chapters, Mimi Watford confronts Caswall who has finally succumbed to madness. He lures her onto the roof of Castra Regis House as a storm approaches and shows off his kite, despite the thunderheads building in the sky. Arabella, who had been stalking Mimi, watches from nearby and steals some of the wire holding the kite, apparently unspooling it all the way back to her house. When Mimi discovers Caswall has locked her onto the roof she shoots off the lock with a gun Adam gave her for her protection and flees home.
Adam convinces her to go back outside with him where they witness the following events: a massive thunderstorm breaks over Castra Regis House, a lightning bolt is grounded by the kite and demolishes the Castra Regis tower; it then travels through the wire Arabella had run to Diana's Grove and ignites Adam's dynamite, which pulverizes the White Worm and destroys the house and Arabella at the same time. Adam, Sir Nathaniel, and Mimi Watford survive the ordeal. In the closing scenes, Adam and Sir Nathaniel survey the destruction. (In the 1925 edition: After this, Adam and Mimi Watford are married.)