Helen, a severely indebted British woman, travels from France to the family castle of her German friend Barchen. Barchen's young nephew Peter was recently orphaned, and Barchen needs a governess to care for him until he can make plans for his future. If Helen helps him out, Barchen promises to cover her debts. Helen meets Peter's nurse Mimi, a beautiful Frenchwoman with a startling lack of teeth. The first night Helen is there, Barchen shows her around the castle. He explains his family's long history guarding the salt mines upon which the castle is built. He firmly warns her to stay away from the water. When pressed on the strange shape which floats in the middle of the lake, Barchen says it is nothing but a log and hurries inside.

Barchen leaves the castle as early and quickly as possible. Helen is left with a disobedient Peter, a nearly silent Mimi, and a passively hostile house staff. Peter has a tendency to run off and get lost in the castle, and Mimi refuses to help search for him. After she searches every floor of the exceedingly dirty castle, Helen finds Peter in the cellar, digging his fingers under a small gap in the door to the crypt and crying for his mom. He pulls a bone from under the door, similar to all the bones Helen found throughout her day in the castle. When she tries to pull him away, he acts ashamed but shoves his fingers into her mouth. Helen falls and gashes her shin open on the stone steps. Peter then lets slip that his parents have been dead for years. Helen begins to wonder if Barchen was telling her the truth about the family situation. The other house staff are unwilling to answer any questions directly; they evade her and give noncommittal answers.

Helen gets drunk that night. When she looks over the lake, she sees another log has joined the previous one. She accidentally drops a bottle of wine down to the rocks below, and the "logs" turn to look at her. She sees that they are serpents like the ones carved on all the furniture around the house. Determined to find the truth, Helen breaks into the crypt and finds a series of salt caves with a pool at the far end. Embedded in the walls are a series of human-sized holes. Peter, who snuck in with Helen, runs towards the holes and drags from them a large maggot-like creature. He attempts to eat it, but Helen throws it into the pool. Helen drags Peter from the room, after which he promptly shoves his slime-covered fingers into her mouth. Something shifts inside Helen's mind. She becomes obsessed with the taste and scent of whatever lurks behind the crypt.

The next morning, Helen finds Mimi beating her head bloody against the crypt door, and when Peter appears at the top of the stairs, Mimi lunges at him and almost drags him outside. Helen grabs him at the last moment and Mimi dives into the lake, where she is immediately swallowed by the serpent. Over the next three days, Helen slowly devolves into a state of obsession over the scent of the crypt and rhythm of the serpents in the lake. Her teeth begin to fall out, she loses the ability to write, and soon she becomes a mirror of Mimi.