Anne, a subdued and dutiful pastor's wife, feels trapped in her monotonous small-town life and her passionless marriage to her husband, Jakob. Her spirits are lifted when she meets an old flame, Tom, who rekindles her sense of adventure. The two decide to explore an old mill, but their escapade takes a terrifying turn when they encounter a malevolent, supernatural force. Anne is attacked by this sinister presence, which starts to unleash unsettling changes in her behavior and desires.

As Anne struggles with her new, dark impulses and navigates the frightening emotional and physical transformations erupting within her, Jakob notices his wife's dramatic shift. Tension mounts as he attempts to understand the cause of her drastic evolution, leading him into a dangerous confrontation with the evil threatening their town. The story builds as Anne and Jakob grapple with their shifting relationship and face a looming darkness that tests their faith and love.