Richard, an esteemed archaeologist and professor, resides with his wife, Jules, and their asthmatic young son, Owen, on his late father's secluded farm located in the moorlands. One day, Owen mentions to his mother strange whistling sounds from an individual named Jack Grey. Richard, troubled by this revelation, reacts with hostility and suspects an old family friend, Gordon, who occasionally babysits Owen when Richard is occupied with work and Jules is away. Haunted by his own childhood trauma of parental abuse, Richard insists that Gordon should no longer have contact with his son, fearing he may be reciting sinister folk tales.
During a school sports day, while Owen participates in a soccer game, Richard and Jules find themselves quickly drawn to alarming screams from a gathering crowd. They discover a young girl’s pony has been brutally attacked and blinded, leaving Owen isolated nearby, clutching a bloodied, sharpened stick in his small hands.
In hopes of understanding Owen’s distress, Richard and Jules seek the help of a psychiatrist. After a series of tests yield no clear issues, Richard dedicates more time to Owen, leading to a return of his gentle demeanor. However, as Richard is away at work one day, Jules finds herself entranced by the ominous mist enveloping the farmyard. When her trance breaks, she discovers Owen lying inactive in the doorway, unresponsive. In a frantic state, she contacts Gordon for help, and together they rush Owen to the hospital.
Upon Richard's arrival at the hospital, he is met with devastating news: his son has succumbed to a severe asthma attack. Overwhelmed by grief, both parents struggle to cope, with Jules falling into a deep depression, nearly incapacitated. Her sister, Harrie, comes to offer support. In an effort to aid his grieving wife, Gordon brings in a local psychic, Mrs. Forde, who attempts to cleanse their home and assures Jules that Owen has found peace. A candle's abrupt flickering adds an eerie atmosphere to their surroundings, which, while unsettling, marks the beginning of Jules’ slow recovery as she redecorates Owen’s room with Harrie's assistance.
Meanwhile, Richard discovers his father's journal, which reveals the existence of a potential spirit entrance within the farmyard, where the essence of nature can transcend into their world. Driven by this intriguing prospect, Richard begins an archaeological dig in hopes of uncovering what his father believed to be true. Disturbingly, the journal also contains his father's dark intentions to sacrifice a young child to the spirit.
As he digs deeper, Richard stumbles upon the bones of a hare. Initially indifferent, he takes these bones to his study, but when an unexplainable organic matter begins to envelope them, he seeks the opinion of a colleague at his college. Dismissed by the colleague and later ordered to take a sabbatical to recuperate, Richard becomes frustrated and storms out.
Within days, the organic matter evolves, transforming the hare's bones into a living creature. Jules becomes infatuated with this creature, dubbed Jack Grey. As Richard uncovers more from the dig, revealing roots from an enormous tree linked to the spirit entrance, the narrative spirals into chaos. In a shocking turn of events, as Richard's colleague arrives at the dig and poses a threat to both the discovery and the hare, Jules resorts to violence, fatally stabbing him.
Gradually, Jules convinces Richard to accept the hare as their new child, even confessing that she sensed Owen's life-threatening asthma attack and believes their lives would be improved without him. This admission leads to a dark climax when Harrie, trying to intervene in their twisted familial dynamic, becomes another victim as Richard murders her. With Harrie's lifeless body nearby, the couple finds solace in their shared madness, nurturing the hare, which they now embrace as their own, as Jules lovingly tends to it, evoking an unsettling atmosphere of loss, love, and horror in their once serene farmhouse.
