In 1887, Edith Cushing (Mia Wasikowska), the young daughter of wealthy American businessman Carter Cushing (Jim Beaver), is visited by her mother's ghost who warns her, "beware of Crimson Peak." Fourteen years later, Edith is now a budding author who prefers penning ghost stories to writing the romance novels that her editor wants. She meets Sir Thomas Sharpe (Tom Hiddleston), an English baronet who has come to the United States seeking investors, including Edith's father, for his clay-mining invention. Disdaining privileged aristocracy and unimpressed with Sharpe's prototype and previous failures to raise capital, Cushing rejects Thomas's proposal. Edith notices that Thomas and his sister, Lucille (Jessica Chastain), wear expensive but outdated and somewhat-frayed fashions. Shortly after, Edith once again is visited by her mother's spirit bearing the same warning. Sir Thomas is determined to persuade Mr. Cushing to change his mind. However, when he and Edith become romantically attached, Cushing and Edith's childhood friend, Dr. Alan McMichael (Charlie Hunnam), disapprove. Mr. Cushing hires a private detective who uncovers unsavory facts about the Sharpes. Mr. Cushing confronts the siblings and bribes them into returning to England. As Cushing insisted he do, Sir Thomas abruptly and cruelly ends his and Edith's relationship, but the next morning, he sends her a note explaining his actions. Not long after, Mr. Cushing is brutally murdered, though his death is ruled accidental. Edith and Sir Thomas eventually marry and return to England. They arrive at Allerdale Hall, the Sharpes' dilapidated mansion, which sits atop a red clay mine. As Edith settles in, she finds that Lucille acts somewhat cold toward her while Sir Thomas remains physically distant. Edith is left confused and uncertain by their behavior. Gruesome ghosts begin appearing to Edith throughout the mansion. To help calm Edith, Sir Thomas takes her into town. After being snowed in for the night, they finally consummate their marriage. Lucille angrily lashes out when they return the next morning, frightening Edith. By the time Sir Thomas mentions that the estate is also referred to as "Crimson Peak," due to the warm red clay seeping up through the snow, Edith is growing increasingly weaker and coughing up blood. Edith explores the mansion and begins piecing clues together, discovering that Sir Thomas previously married three wealthy women who were fatally poisoned for their inheritances. She realizes she, too, is being poisoned. She also discovers the siblings have had a long-term incestuous relationship, resulting in a sickly infant that died. Lucille murdered their mother after she had discovered her children's incest. Sir Thomas inherited the family manor that, like many aristocratic estates of the era, is no longer profitable; the Sharpes are virtually penniless. The brother and sister began the "marriage and murder" scheme to support themselves and to fund Thomas's inventions. Back in the United States, the detective that Mr. Cushing had hired tells Alan what he uncovered about the Sharpes, including Thomas's multiple marriages and Lucille's time in a mental institution. Realizing Edith is in danger, Alan arrives at Allerdale Hall to rescue her. Lucille stabs him in the left armpit, then demands that Sir Thomas finish him off. Sir Thomas, who has fallen in love with Edith and does not want her harmed, inflicts a second, non-fatal stab wound to Alan before hiding him in the cellar. Lucille forces Edith to sign a transfer deed granting the Sharpes ownership of the Cushing estate and also confesses to her that she was the one who murdered Edith's father. After Edith signs the deed, she stabs Lucille in the chest with a pen and tries to flee. Sir Thomas burns the deed and promises to protect Edith so that she and Alan can escape. Lucille, jealous over Sir Thomas falling in love with Edith, murders him in a rage. She then pursues Edith. Aided by Sir Thomas' ghost, Edith kills Lucille with a shovel. Sir Thomas bids Edith a silent farewell as his spirit departs. Edith and Alan are rescued, and Lucille's ghost now haunts Allerdale. The end credits imply that Edith has written a novel titled Crimson Peak based on her experiences.