A farm girl signs on as governess in a gloomy mansion.

In 1844, lovely, young Miranda Wells lives on a farm near Greenwich, Connecticut and often escapes her mundane existence through daydreams of a romantic, luxury-filled life. Miranda is thrilled when her mother Abigail receives a letter from Nicholas Van Ryn, a distant cousin whose extensive lands and mansion, Dragonwyck, are the envy of his fellow Hudson River landholders. Abigail explains to Miranda that Nicholas is descended from the original Dutch patroons, but despite his wealth, she is wary of his request that one of her daughters serve as governess to his eight-year-old daughter Katrine. Miranda's religious father Ephraim is opposed to the idea, until Miranda shows him a Bible passage that convinces him. Soon after, Ephraim and Miranda meet Nicholas, who passes Ephraim's inspection despite his arrogant attitudes toward the tenant farmers working the Van Ryn lands. Upon reaching Dragonwyck, Nicholas introduces Miranda, with whom he has shared an instant attraction, to Katrine and Johanna, his gluttonous wife. One afternoon, Miranda and Katrine hide in the woods to watch the kermess, an annual ceremony at which Nicholas receives the rents of his tenants, and are discovered by Dr. Jeff Turner. Jeff expresses interest in seeing Miranda again, but warns her that he dislikes Nicholas, whose autocratic policies are antipathetic to his own democratic principles. At the kermess, farmer Klaas Bleeker asserts that his family has paid Nicholas the value of their land many times over and refuses to pay more rent. Nicholas then evicts him, after which Jeff prevents Klaas from attacking Nicholas, while continuing to defend Klaas's political position. At a ball hosted by Nicholas that evening, some of the high society guests snub Miranda due to her humble background. Soon after, Jeff comes to Dragonwyck to ask for help in defending Klass, who has been falsely accused of murder. Nicholas is disinterested at first but then promises to help and asks Jeff to examine Johanna, who has a head cold. Jeff assures her that the cold will pass, then joins Miranda for dinner, while Nicholas, who has brought his favorite oleander plant to Johanna's room to cheer her, stays to give her some cake. Later that night, Johanna dies suddenly, much to Jeff's astonishment. Nicholas meets Miranda downstairs and tells her that he was never happy with Johanna, who could not bear him a much longed-for son. Miranda can no longer fight her feelings either and the couple admit their love, although Miranda returns to Connecticut the next day. At the farm, Ephraim is irritated by Miranda's distant behavior, but her reserve is explained two months later when Nicholas arrives to ask for her hand in marriage. Ephraim and Abigail reluctantly consent, and a few months later, Nicholas is away on business when Miranda discovers that she is pregnant. Despite their quarrels over Nicholas' lack of religious faith and his fury over Miranda's hiring of a lame maid, Peggy O'Malley, Nicholas is thrilled by the news. Nicholas' joy is tempered by new laws permitting tenant farmers to buy their lands, but despite the threat to his historic rights, he anticipates passing Dragonwyck on to the baby, who he is sure will be a son. Nicholas asks Jeff to attend Miranda when she gives birth, and although Jeff is pleased to see Miranda, whom he still loves, he must tell her that her newborn son has a defective heart. Miranda has the baby baptized before he dies, but Nicholas is enraged and heartbroken by his son's death. As the months pass, Nicholas grows more distant and embittered, and informs Miranda that he has become a drug addict. Fearing for her mistress' life, Peggy begs Jeff for help and reveals that Nicholas has brought the oleander plant to Miranda's room. Jeff rushes to Dragonwyck and there accuses Nicholas of poisoning Johanna with oleander and attempting to do the same to Miranda. Much to Miranda's horror, Nicholas admits his crime and begins to fight with Jeff. Nicholas is knocked out during the struggle, but after Jeff and Miranda leave, the addled Nicholas staggers to the kermess grounds and calls for the farmers to bring their tributes. As Nicholas waves his pistol, the farmers assemble with Jeff, Miranda, the mayor and sheriff, and Nicholas is shot when he takes aim at Jeff. The farmers doff their caps as Nicholas dies, and later, Miranda prepares to return to Greenwich, which she now realizes is her true home. Jeff bids her farewell and promises to visit her the following week.