Dr. Eric Price, a struggling physician with a growing dependency on drugs and alcohol, is tasked by the Winchester Repeating Arms Company to evaluate the mental state of Sarah Winchester, the eccentric widow of the company's founder. Sarah, who lives a reclusive life in an elaborate, ever-expanding California mansion, believes she is haunted by the spirits of those killed by the firearms produced by her late husband's company. Upon arriving at the mansion, Price encounters an intricate maze of rooms, staircases leading to nowhere, and ongoing construction work. Initially skeptical of Sarah's claims, he begins to experience strange phenomena that challenge his understanding of reality.

As Price delves deeper into the mansion's mysteries, he learns that Sarah's construction efforts are driven by a need to appease vengeful spirits. Each room is built according to the instructions provided by these spirits, who seek resolution for their untimely deaths. Impressed by her meticulous dedication yet unnerved by the mounting supernatural occurrences, Price grapples with his own haunting past while striving to determine whether Sarah's beliefs are rooted in madness or genuine contact with the supernatural. Events escalate dramatically, forcing him to confront forces beyond his comprehension and compelling Sarah to address the consequences of the Winchester legacy.