 Set in Cumbria, England, in a crumbling mansion in a largely rural and mountainous region of northern England in the early 20th century, young author Edith Cushing (Mia Wasikowska) falls in love and marries Sir Thomas Sharpe (Tom Hiddleston) but then discovers that her charming new husband is not who he appears to be. His home harbors ghostly, mysterious entities and secrets, which he and his sister, Lady Lucille Sharpe (Jessica Chastain), fiercely try to hide. Edith: [narrating] Ghosts are real, this much I know.
 At the start of the film (in the year 1887) a young Edith Cushing, the daughter of self-made industrialist, Carter Cushing (Jim Beaver), is introduced.
 Edith: [narrating] The first time I saw one I was 10 years old. It was my mother's. Black cholera had taken her. So Father ordered a closed casket, asked me not to look. There were to be no parting kisses. No goodbyes. No last words. That is, until the night she came back.
 Edith has a terrifying encounter with her mother's restless spirit. Wrapping her dark, decaying fingers around young Edith's shoulder as the girl shudders in her bed trying to will away her terror, the ghost whispers a warning: "Beware of Crimson Peak."
14 Years later, when Edith is a young woman living in Buffalo, New York she aspires to write about the ghosts she sees and encounters, though it is implied that her mother's is the clearest and most intimate encounter she has ever had.
Later at a social gathering, women are buzzing about a new arrival in town, an aristocrat from Cumbria.
Society Girl: It seems he's a baronet.
Society Girl: What's a baronet?
Society Girl: Well, an aristocrat of some sort.
Edith: A man that feeds off land that others work for him. A parasite with a title.
Society Girl: This parasite is perfectly charming and a magnificent dancer. Although, that wouldn't concern you, would it, Edith, our very young Jane Austen?
Edith is pressured into writing about romance as it is expected of her gender. Having dinner later with her father, Carter presents her a writing pen as a gift, but she insists she will use a typewriter to write her story. Edith begins writing the story at Carter's office when it passes the eye of the visiting aristocrat seeking funding for an invention of his own from Edith's father.
Edith: You're not late, are you? He hates that.
Thomas: Not at all. In fact, I'm a little early.
Edith: Well, I'm afraid he hates that too.
The aristocrat is Sir Thomas Sharpe of Allerdale Hall, which is located atop a mine for red clay ultisol that is used in brick-making and rendering for iron ore. Thomas credits her as a gifted writer and story-teller, points out it being about ghosts.
Thomas: You see, where I come from, ghosts are not to be taken lightly.
In a conference room full of businessmen, Thomas gives a presentation to pitch his clay harvester. He brings along a model of his machine, claiming it will revolutionize clay mining. Being wary of aristocracy, the New York gentleman, Carter, turns away the Cumbria-born Sharpe with extreme distaste.
Carter: I started as a steel worker, raising buildings before I could own them. My hands, feel them. Rough. The reflection of who I am. Now you, sir, when I shook your hand, you've got the softest hands I've ever felt. In America, we bank on effort, not privilege. That is how we built this country.
That evening, Edith declines to go to a ball with her father and her childhood friend, Dr. Alan McMichael (Charlie Hunnam). Moments later, she is visited by the spirit of her mother once again warning her of Crimson Peak.
Alan: As I recall, Edith takes a dim view of social frivolity.
Edith: You lads enjoy the party. [whispers to Alan] Don't let him drink too much.
Moments later, she is on her bed looking at some books when her bedroom door opens on its own. Edith is visited by the spirit of her mother once again warning her of Crimson Peak.
Feeling rejected by Carter, Thomas turns his attention to Edith. He admits he waited until after Carter left before visiting her. He convinces her to attend the ball with him.
Everyone's eyes are on Thomas and Edith as they enter the ball just after Lady Lucille Sharpe, Thomas' older sister, gives a piano performance. Thomas demonstrates to the crowd how to do the Waltz and picks Edith for his dancing partner. Charmed by his ways, Edith begins a romance with Sir Thomas that is watched closely by Alan.
The next day, Carter hires an investigator to look into Thomas and Lucille's past. Meanwhile, Edith and Lucille are at a park where the ground is covered with dying butterflies.
Lucille: They're dying. They take the heat from the sun, and when it deserts them, they die.
Edith: How sad.
Lucille: No, it's not sad, Edith. It's nature. It's a world of everything dying and eating each other right beneath our feet.
Edith: Surely there's more to it than that.
Lucille: Beautiful things are fragile. At home, we have only black moths. Formidable creatures, to be sure, but they lack beauty. They thrive on the dark and cold.
Edith: What do they feed on?
Lucille: Butterflies, I'm afraid.
Sometime later, the investigator returns with documents of their dark past. Carter confronts Thomas and Lucille and demands they leave town immediately. Thomas agrees on the promise Carter does not tell Edith about what he found on their past. Thomas comes up with a harsh farewell as requested by her father to break her heart.
Thomas: There is nothing to hold us in America.
Edith: I see.
Thomas: Your novel. I read the new chapters and having delivered it in the morning. Will you still like to know my thoughts?
Edith: If we must.
Thomas: It's absurdly sentimental. The aches that you describe with such earnestness, the pain, the loss. You clearly have not lived it at all. In fact, you only seem to know what other writers tell.
Edith: That's enough!
Thomas: You insist on describing the torments of love when you clearly know nothing about them. I'm not done yet! What do you dream on? A kind man? A pure soul to be redeemed? Affection? Affection has no place in love, Edith. I advise you to return to your ghosts and fancies, the sooner the better. You know precious little about the human heart or love or the pain that comes with. You are nothing but a spoiled child!
Mysteriously, Cushing is brutally killed, his skull crushed in. Meanwhile, Edith receives a letter from Thomas confessing that his father pressured him to deceive her and leave. She races to his hotel to be told he has already checked out, but she turns around and sees Thomas has stayed.
Thomas: I cannot leave you, Edith. In fact, I find myself thinking about you even at the most inopportune moments of the day. I feel as if a link exists between your heart and mine, and should that link be broken, either by distance or by time, then my heart would cease to beat and I would die.
In the wake of her father's untimely death, Edith seeks solace in her romance with Sir Thomas who gently and tenderly seduces her into leaving New York for his own home in England to escape the ghosts of her past. Remembering the horror of being visited by her mother, Edith immediately agrees and sets sail for England. McMichael is wary of this arrangement and makes his own way to England later.
Once at Allerdale Hall, a brilliantly clad but dilapidated mansion teetering on the red mire beneath it and crumbling under the snow and rain above, is revealed to be the Sharpe family home.
 Its unofficial name is Crimson Peak due to the red clay beneath it, though this is not mentioned to Edith at first, so the warning is not heeded. Edith: [entering his mansion] Goodness. How many rooms are there? Thomas: I don't know. Would you like to count them?
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