In 1865, Katherine is trapped in a loveless marriage to an older man, Alexander Lester, and the couple reside at the estate of Alexander’s father, Boris, in a remote corner of rural Northumberland. Katherine is forced to cling to a rigid daily routine and is largely forbidden from leaving the house, a life that feels more like a cage than a home. Boris scolds her for not bearing a son, while Alexander’s own libido appears to be directed more at watching her naked body than at any real emotional connection. The atmosphere is claustrophobic, with power and control weighing heavily on Katherine as she navigates a household built on hierarchy and fear.
When both Boris and Alexander must depart separately for business, Katherine finally finds her first moment of freedom, albeit a dangerous one. She frees Anna, the housemaid who is being mistreated, and she ventures beyond the confines of the manor. It is during this quiet, uneasy exploration that Katherine becomes drawn to Sebastian, a laborer she encounters on the grounds, and their relationship quickly deepens into an illicit affair. The affair marks a turning point, shattering the fragile stability of the estate and exposing the brittle loyalties that hold the household together. A local priest warns Katherine, but she brushes him aside, choosing secrecy over conscience as the affair intensifies.
Boris returns home sooner than expected, noticing tensions and misattributed changes. He suspects Anna, accuses her of drinking his wine, and then harshly asserts his dominance: he orders Anna to lie on the floor on all fours, treating her as an animal if she fails to behave. Katherine remains silent, watching the cruelty unfold. When Boris learns of the affair, he brutally beats Sebastian and confines him to a stable, and he strikes Katherine again when she presses for Sebastian’s release. In a calculated act of defiance, Katherine poisons Boris’s food and calmly carries on a mundane conversation with Anna as he suffocates in the next room. The household settles into a hush of fear as Anna is silenced and Boris is buried without suspicion.
With Boris gone, Katherine assumes control of the estate and continues her open liaison with Sebastian. The couple’s audacity grows bolder, and the emotional and sexual tension between them intensifies. One night, when Alexander unexpectedly returns and reveals that he knows about the infidelity, Katherine summons Sebastian to the marital bed and they resume their affair under his watchful presence. A brutal confrontation follows, and Katherine kills Alexander in a struggle. The two lovers discard the evidence—burying Alexander’s body in the woods and killing his horse—and they manage to avoid immediate punishment as suspicion shifts away from them, especially since Anna remains mute.
Sebastian begins to adopt the manner of a lord, enjoying the privileges of status that come with Katherine’s ascendancy. The estate becomes a stage for their audacious, dangerous partnership, even as new tensions simmer beneath the surface. Agnes arrives with a young boy named Teddy, claiming he is the product of an affair between Alexander and her daughter. Katherine reluctantly shelters them, and Sebastian resents the new dynamic, retreating to the outbuilding for periods of isolation. Katherine, meanwhile, discovers she is pregnant, and she starts to form a fragile bond with Teddy, a child who becomes both a symbol of possibility and a tether to the consequences of their actions.
Teddy’s presence unsettles the household further. After Teddy goes missing briefly, Sebastian rescues him from a perilous fall by a waterfall, and he brings him back to the house. He confesses privately that he once considered pushing Teddy into danger, a revelation that unsettles Katherine. In a moment of calculated pragmatism, she persuades Sebastian to stay, promising to fulfill any wishes in return. Yet the village grows suspicious, and Teddy’s grandmother slips into sleep, creating an opportunity for Katherine and Sebastian to smother Teddy in his bed—an act that seals their grim defiance.
Sebastian retreats into the woods as guilt gnaws, while Katherine contends with the authorities and the truth. The village doctor remains skeptical of the story, and Sebastian eventually returns with a confession, though Katherine quickly turns the narrative back on him, accusing him of orchestrating all the murders with Anna. With Anna mute and unable to defend herself, the weight of the accusations falls on Sebastian, who is taken away along with Anna. The remaining servants depart the estate, leaving Katherine to face the future alone, her unborn child a stark reminder of the violence and ambition that have rewritten her life.
