The story begins with Adam Salton, a young man visiting his great-uncle, Richard Salton, in the eerie countryside of Derbyshire. Soon after his arrival, Adam hears unsettling stories of a legendary creature, the White Worm—a mythical, ancient serpent said to lurk beneath the land.
Adam's curiosity leads him to Castra Regis, the estate of the enigmatic Edgar Caswall, a man who recently returned from Africa with his unsettling servant, Oolanga. Caswall is not only charismatic but also a master of mesmerism and mind control, using his power to dominate the villagers, particularly women. Oolanga, while silent, is a foreboding presence, amplifying Caswall's control and mysticism.
As Adam tries to acclimate to this strange world, he encounters Lady Arabella, the beautiful and dangerous owner of Diana’s Grove, an estate tied to the ancient legends of the White Worm. Unbeknownst to Adam, Lady Arabella is a half-human, half-serpent creature—a living embodiment of the White Worm—who uses her seductive charms and supernatural abilities to manipulate others, including Adam himself.
Lady Arabella and Caswall both vie for control of the ancient powers connected to the White Worm, with Arabella using her shape-shifting abilities to physically transform into a massive serpent during key moments of the plot. Meanwhile, Caswall’s mastery of hypnosis and mental manipulation draws in the village women, fueling his twisted desires for control. Arabella and Caswall form an unholy alliance, but tension builds as each seeks to manipulate the other to achieve their own ends.
As the plot unfolds, Adam becomes romantically involved with Lilla Watford, a local woman who is targeted by Caswall's sinister advances and Arabella's supernatural manipulations. Lilla resists Caswall’s hypnotic control, leading to a series of dangerous encounters between Caswall, Lady Arabella, and Adam. Arabella’s true nature is fully revealed as she plans to use Lilla as part of a ritual sacrifice to resurrect the full power of the White Worm.
In the climactic battle at Diana’s Grove, Adam, along with Sir Nathaniel, leads a group of villagers to stop the dark ritual orchestrated by Lady Arabella. As the fight unfolds in the depths of the cavernous lair, Lady Arabella fully transforms into the White Worm, using her massive serpentine body to attack Adam and the villagers. Caswall, also seeking to control the Worm’s power, is betrayed by his servant Oolanga, who stabs him in a fit of rage. Caswall kills Oolanga but succumbs to his injuries shortly afterward.
In the final moments, Adam stabs Lady Arabella with a ceremonial spear, weakening her. As the lair collapses around them due to the explosive charges set by Sir Nathaniel, Lady Arabella is buried under the debris, bringing an end to the threat of the White Worm. Adam, Sir Nathaniel, and the remaining villagers escape as the ancient evil is seemingly destroyed.