Twins Billy and Dara’s mother has been missing for twenty-eight days when a woman appears in their driveway. Billy notices she looks very much like their mother, but her deeper voice sets her apart. The woman introduces herself as their aunt, Lydia Leigh, but asks them to call her Aunt Lydie. Billy is confused and says he thought his mother was an only child, but Dara seems to recall their mother having a sister. Upon her inquiry about her sister, Billy tells Lydie their mother has been missing for almost a month. Seeing Lydie’s distress, Dara leads her inside, and the three have dinner. Uncomfortable with his aunt’s cold eyes and her distaste of doctors—the career Billy wishes to follow—Billy hints at his aunt leaving before the rainstorms come. However, Dara admonishes Billy’s harshness and welcomes Lydie to stay longer. Lydie then offers to do Dara’s hair.

During Lydie’s stay, Billy stays in his room, sometimes reading his anatomy textbook. He recalls his mother’s supportive reaction when he told her he wanted to be a surgeon, buying him a scalpel set and model skeleton. She told him he could dissect specimens, but not the botflies in the chicken coop because they are bad luck. Lydie spends all her time in Dara’s room, the two chatting away. Billie begins to think his mother didn’t disappear, but rather ran away when she saw trouble on the horizon. When Dara comes down for dinner on the third night of Lydie’s stay, Billy notices she has dark circles under her eyes but that her mood is strangely high. In the morning, the rain clears a little, and Billy asks Lydie if she’ll be leaving. They verbally spar for a moment before Lydie reveals she has come to their house in search of something their mother has hidden. She says it is a piece of her she cannot be whole without. As she leaves, she slashes Billy’s face with a fingernail. Although the action is painful, it leaves no wound.

Later, Billy waits for Lydie and Dara to come down for dinner. Dara finally appears after midnight, her frame skeletal and her nightgown soaked in sweat. She greedily drinks water and reassures Billy their mother had discussed Lydie before, saying their mother had told her they would have to brace themselves for an unstoppable force to do its work. Billy tries to calm himself on the porch, but Lydie is already there. She tells him she won’t leave, asserting that mothers share secrets with their daughters. Billy realizes she is just like the botflies, filling an empty place with darkness and evil. He snaps that his mother will never return, but Lydie unleashes some force upon him, causing him to convulse and fall to the floor. Desperate to be rid of her, he lures her around the side of the house, promising it’s where his mother kept her most precious items. Lydie follows, and Billy takes a hatchet from the woodpile and dismembers her. Afterwards, he puts the pieces in the chicken coop and lights them on fire.

Once he is finished, Billy rushes back to the house to get Dara. He finds her body is covered in what seems like strands of hair. She tells him something is hurting her, like it’s moving inside her chest. Billy brings her to Lydie’s car, along with the hatchet and his surgical tools. The hatchet is for the foes the two may meet on their journey, as well as for handling their mother if she abandoned them. The tools are for removing anything Lydie may have left inside both of them. When Dara expresses concern for Lydie’s absence, he eases her worries and tells her that Lydie is staying behind to maintain the home. As they leave, an angry wail shakes the house. Billy heads for the river near the town, remembering it as the direction their mother had been witnessed disappearing towards.