Beatrice Lacey is the daughter of the Squire of Wideacre, an estate situated on the South Downs, centred around Wideacre Hall. Devoted to her father, at the age of five years she falls in love with the estate and decides to stay there forever. At 11, her dreams are shattered when she learns that her brother absent Harry will inherit the estate, and that she will make a good marriage and leave. Young Beatrice begins an affair with Ralph, the gamekeeper's son, who lives with his mother, Meg, a village witch, in a cottage on the estate. Harry returns and discovers them, ending the relationship. Threatened by Harry's presence, Beatrice agrees without thinking when Ralph reveals his intent to take the estate for the two of them. She realises too late what Ralph has unmanned, and before she can stop him he murders her father and makes it look like an accident. Enraged by the sight of her father's corpse, guilty, and afraid that if Ralph were ever caught he implicate her, Beatrice decides she cannot allow him to continue living on Wideacre. She lures him over a mantrap and leaves him for dead. To her dismay, she later discovers that he has escaped—maimed but alive—with his mother's help. Knowing will someday seek revenge, Beatrice becomes more callous, manipulative and ruthless.
Beatrice teaches Harry how to run Wideacre, but soon her position is threatened by Harry's attraction to their neighbour's stepdaugher, Lady Celia Havering. Beatrice handily seduces Harry to gain control of him, and befriends the sweet and innocent Celia. Harry marries Celia with the blessing of Beatrice, who accompanies them on their honeymoon to France. Beatrice discovers she is pregnant with Harry's child, lying to Celia that the child is the product of a rape. Celia decides to pass the child off as her own, sending Harry back to England and later writing to him with the "good news". Beatrice gives birth to a girl, whom Celia names Julia. Despite a newly assertive Celia taking her place as Harry's wife and Lady of Wideacre, Beatrice secures her hold over her brother.
At the peak of her power, Beatrice is attracted to the intelligent and provocative young Dr John MacAndrew. Determined to stay on Wideacre, she refuses his marriage proposal; finding herself pregnant again by Harry, she agrees to marry John, who in turn agrees to live at Wideacre. Beatrice gives birth to a boy whom she names Richard, and intends to pass off as John's child. As a doctor, however, he can see immediately that the baby is not premature. Disillusioned, John refuses to believe Beatrice when she says that she was raped but that her love for him is not a lie. He begins to drink to forget her betrayal. Harry seduces Beatrice and their mother discovers them. She faints from the shock, and in a catatonic state she mutters over and over "I only came to get my book ... Harry, Beatrice, no!" Beatrice knows her mother will ultimately reveal her secret, so she manipulates Celia into inadvertently overdosing her mother on the laudanum John has prescribed. Beatrice and Harry's mother dies; John realises what Beatrice has done, and also now suspects her relationship with Harry. Before John can come through on his threats to ruin her, Beatrice uses his drinking to systematically destroy him and his reputation. When she has him dragged off to an insane asylum, his screams that she is an incestuous whore and a murderess fall on deaf ears.
With John out of the way and his£200,000 fortune under her control, Beatrice coerces Harry to go along with her scheme to marry "cousins" Julia and Richard to each other and make them joint heirs to the Wideacre estate. In need of more money to complete their plan, Beatrice and Harry mortgage the estate and begin to enclose the common land. As this strips the villagers of places to graze their animals and raise their own vegetables, it incites anger and resentment on the estate. Beatrice, intent on her plans, does not care. Realising what is happening, Celia frees John from the asylum, bringing him back to Wideacre and managing to restore his medical reputation. John and Celia do their best to help alleviate the villagers' poverty and depravation, in contrast to the increasingly corrupt and ruthless Beatrice and Harry.
Word comes that "The Culler", a shadowy outlaw who is against enclosure and the aristocracy, is heading for Wideacre. Knowing that the Culler is her first love Ralph, Beatrice is both afraid and desirous of his vengeance. Harry discovers that Julia is Beatrice's daughter (though not that he is the father of Julia or Richard). Finally recognising the enormity of Beatrice's crimes and destructive nature, Celia calls her out and leaves, with Harry and Julia in tow. John takes Richard and leaves as well, his only remaining desire being to save Celia and the children from the corruptive influence of Beatrice's wickedness. Harry dies of a heart attack en route. Left alone, Beatrice knows that the arriving villagers have to come to burn down the Hall and kill her, but she does not care. She is overjoyed to see Ralph, though the last thing she sees is the knife in his hand. She welcomes her death, understanding that it is justice and her only hope of redemption.