Following the death of their father, Henry and his sister Rachel travel to their childhood home, Monkshood, located in the English countryside. Rachel agreed with their father she would inherit the estate and Henry would inherit its contents. When they arrive, Henry visits the cellars of the house and recounts some of his memories there. Monkshood was built on an old priory, and Henry and Rachel used to play in its cellars when they were little. However, Rachel was always braver and would often go alone, later resulting in her mysterious sleepwalking habit. To ensure she would not sleepwalk to the cellars, her father sealed them. Henry also reveals his father would take him to the basement to physically abuse him.
After a night at the estate, Henry wakes to find Rachel smashing open the plaster over the cellars’ entryway. They attend the funeral, during which a neighbor tells Henry Monkshood had not been built over a priory at all. Confused but apathetic, Henry ignores her. He and Rachel speak with the solicitor, who tells them their previous arrangement was reversed. Their father has left Henry the estate and Rachel its contents. Devastated, Rachel tells Henry their father is giving him to Monkswood, not Monkswood to Henry. They meet in the woods outside the house. Rachel reveals to him she wanted to burn the home, not sell it, because something is living in the cellars. She knew of its existence from the times she would visit the cellars alone, calling it “Slipper” for the sounds of the beatings their father would give Henry with a slipper. Henry remembers an instance after one of these beatings when Rachel was comforting him in the woods, ashamed of the intimacy they shared when he felt he had no control over his body.
The two go to the cellar doorway, promising the thing in the darkness a gift. In exchange, the thing must sleep for a long time. The darkness seems to agree, and they descend farther. They come into a room filled with drying skins, some animal, some human. Henry and Rachel hold hands as Henry lights a rag in a vodka bottle and throws it at the thing. They die together, hoping they took the thing with them.