In 1887 Buffalo, N.Y., 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, a budding author, 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 Edith notices that Thomas and his sister, Lucille (Jessica Chastain), wear expensive but outdated and somewhat-frayed fashions.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, her mother’s spirit once again visits Edith, bearing the same warning.
When Sir Thomas 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 to leave town and for Sir Thomas to end his and Edith’s relationship by breaking her heart. The next morning, Sir Thomas 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 is somewhat cold 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 her, Sir Thomas takes her into town. After being snowed in for the night, they finally consummate their marriage. Lucille angrily lashes out after their return, 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. Alan goes to Allerdale Hall to rescue Edith. Lucille stabs him, then demands that Sir Thomas finish him off. Sir Thomas, who has fallen in love with Edith and wants to protect her, 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 and tries to flee. Sir Thomas burns the deed and promises to help her and Alan to 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’ spirit bids Edith a silent farewell and 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.