Anne, Sophie Turner, and her mother-in-law Morwen, a world-weary and unpolished survivalist, scrape by in a late-medieval village while awaiting Seamus, Laurence O'Fuarain's return from the Wars of the Roses. Anne is introspective, gentle, and devout, a quiet contrast to Morwen’s rough pragmatism, yet their bond remains steadfast and affectionate even as hardship tightens its grip around them.
Jago, Kit Harington, Seamus's childhood friend, comes back from war claiming that they deserted—though he insists Seamus was slain by thieves. Morwen remains wary, but Jago’s tale gradually confirms Anne’s dream of Seamus’s death. In the woods, Anne begins to glimpse an armored knight, a figure that gnaws at the edges of her waking life and conscience.
Jago presses his suit on Anne while Morwen warns that he envied Seamus and will steal her away, spinning dark threats about the Knights of Hell should she abandon her duties to Morwen and forget her late husband. When a ship wrecks nearby, the women scavenge its remnants, and Morwen secretly kills survivors. She slits the throat of a pilgrim priest, Friar Penrose, for relics, and a horrified Anne helps hide the body.
As Anne grows closer to Jago, he comforts her that the knight she sees may be real. After a dream in which Morwen becomes a demon, Anne sleeps with Jago. He urges her to leave Morwen and join him, but Anne admits that Morwen both needs and frightens her. Morwen tightens her grip, using Anne as a distraction to kill and rob travelers, and a horse figurine from one victim crops up in Anne’s mind as a sign from the demonic knight.
Morwen threatens Jago with a knife when he visits Anne and fills Anne with terror with tales of the Knights of Hell should she walk away. After an internal battle, Anne drugs Morwen and slips away to spend a bright day with Jago. When Morwen awakens, she encounters the knight herself and follows the lovers. When the knight collapses, clawing at his helmet, she kills him for his purse.
After their night together, Anne confides her envy of a local Lady’s wealth and fears the demon Mammon has been nudging her toward greed; Jago reassures her that such feelings are natural. Anne resolves to live with Jago, defying Morwen, who reacts with rage. The couple’s new life soon fractures when Anne catches Jago in a lie about Seamus’s death. Jago finally confesses that Seamus grew ruthless in war, murdering comrades for loot. He killed an armored knight and stole his helmet; wearing it, Seamus screamed as if burning. Jago fled and later lied about Seamus’s death to win Anne.
In a flashback, Morwen removes the dead knight’s helmet and discovers she killed her own son, his face grotesquely diseased and demonic. She dons the helmet to frighten Anne, but it burns her and she cannot remove it. Anne splits the helmet with a cross, revealing Morwen, now disfigured and begging forgiveness for her sins.
The helmet’s curse stubbornly returns. Anne, unafraid, picks it up despite Morwen’s warnings and appears to Morwen like an angel, even as Morwen goes blind. Refusing both Jago’s and Morwen’s claims of ownership over her, Anne retrieves the helmet from a chest in the woods, and lets it whisper to her, listening to its dark counsel as the story closes.
