In 1887, in Buffalo, New York, Edith Cushing (Mia Wasikowska), the affluent daughter of businessman Carter Cushing (Jim Beaver), encounters the ghost of her deceased mother. This spectral visit carries a foreboding message: “beware of Crimson Peak.” At just ten years old, after her mother succumbed to black cholera, Edith could not comprehend the gravity of this warning.
Fast forward fourteen years, and Edith has blossomed into a promising author with a penchant for ghost stories, much to the chagrin of her editor Ogilvie (Jonathan Hyde), who prefers she write romance novels. A spirited individual, Edith is unafraid to defy the stringent social norms of her time. Aware that her delicate handwriting could reveal her true identity, she decides to type her stories moving forward.
Amidst this backdrop, she encounters Sir Thomas Sharpe (Tom Hiddleston), an English baronet seeking investors for his clay-mining invention. Captivated by Edith’s literary talent, Thomas quickly makes a favorable impression on her. However, Edith’s father, Cushing, dismisses Thomas's venture, asserting that he has worked hard in his life, while Thomas has not. Cushing harbors an uneasy feeling about Thomas from their very first meeting.
As time passes, Edith notices the peculiarities in the Sharpes' clothing—expensive yet outdated—as well as Thomas’s old, hand-stitched coat. Shortly thereafter, Edith is once again visited by her mother's spirit, reiterating the haunting warning. Despite facing disapproval from her father and childhood friend Dr. Alan McMichael (Charlie Hunnam), a respectable ophthalmologist, she slowly finds herself enchanted by Thomas. At a high-society ball, he chooses her over the eligible debutante Eunice (Emily Coutts), revealing her to an exhilarating spotlight she'd never experienced before.
Cushing grows increasingly wary and, in a bid to protect his daughter, hires a private investigator, Holly (Burn Gorman). The investigator uncovers troubling revelations about the Sharpes, leading to a confrontation that forces Thomas to abruptly sever his romantic ties with Edith, cruelly belittling her literary aspirations. Overnight, he regrets his decision after receiving a note from Edith explaining the circumstances that drove his actions.
Tragically, Mr. Cushing is brutally murdered—the event masqueraded as an accident—leading to whispered doubts surrounding his untimely demise. Despite the chaos surrounding her, Edith and Thomas marry and relocate to Thomas's ancestral home, Allerdale Hall, in England. Here, the couple's new life takes a strange turn; while she tries to get accustomed to her surroundings, Edith feels the coldness emanating from both Thomas and his sister, Lucille (Jessica Chastain). Furthermore, she is barred from exploring certain areas of the house, particularly the lower levels wherein the twisted history of the family lies.
As Edith grapples with her mysterious new home, she begins to see horrifying apparitions, and her health deteriorates rapidly. It is during this turmoil that she discovers the sinister secrets of Sir Thomas's past marriages—each ending in deadly poisonings for their wealth. With the unnerving truth of the Sharpes' twisted familial bond and the murder of their own mother surfacing, Edith realizes that she is just another pawn in the siblings' nefarious games.
Back in America, Alan learns shocking truths about the Sharpes through the investigator and, sensing Edith is in peril, rushes to rescue her. Lucille violently stabs Alan, while Sir Thomas, now torn between love and loyalty, intervenes. The tension builds as Lucille forces Edith into signing over her inheritance, unleashing a series of tragic confrontations leading to Edith driving a pen into Lucille’s heart after revealing her involvement in her father’s murder.
In a climactic struggle spurred by jealousy and betrayal, Sir Thomas sacrifices himself to save Edith. In the aftermath of their chaotic lives, the memory of Sir Thomas lingers as Edith, alongside Alan, successfully escapes from Allerdale Hall, now haunted by Lucille's vengeful spirit. Ultimately, the film hints that Edith finds solace in her suffering by penning her experiences into a novel titled Crimson Peak, thus transforming her haunting past into a chilling tale.
