*CUT TO THE CHASE*
NOTE: This spoiler was submitted by Jeremy
The film opens at the infamous Winchester mansion. Young Henry Marriott (Finn Scicluna-O'Prey) is sleeping next to his mother Marian (Sarah Snook) when he suddenly awakens in a trance. Marian goes looking for Henry and finds him at the bottom of the stairs beneath the sealed-off attic. Henry snaps out of it and points upward, saying someone is coming for them.
In San Francisco, psychologist Dr. Eric Price (Jason Clarke) is doing drugs with prostitutes. He is later approached by a man from the Winchester mansion on behalf of his employer, the widow Sarah Winchester (Helen Mirren), who is very disturbed and is seeking out Eric's help in treating her. Eric accepts after learning of his payment.
Eric arrives at the mansion where he meets Marian and Henry before being taken to his room. Because her own husband fell victim to his vices, Marian requests Eric not drink during his stay.
Eric is alone in his room, frequently talking to a picture of his dead wife Ruby (Laura Brent). He carries around a bullet that he was shot with and had it refurbished to have "Together Forever" engraved on it. While taking some of his drugs, Eric is startled by ghostly visions.
Eric joins the family for dinner. He asks Henry about his father, and the boy doesn't seem to care about what happened to him. They are joined moments later by Sarah, dressed in all black.
Sarah starts her sessions with Eric. She knows more about him than he lets on, such as his drug abuse and Ruby. Sarah says she will only be honest with Eric if he is honest with her.
At night, Eric sneaks out of his room and wanders around the mansion without being detected. He goes outside where workers are continuously constructing parts around the house. Eric looks up at the roof and sees Henry walking around with a bag over his head. Eric tries to call attention to this before Henry walks off the ledge. Eric runs and catches Henry before he hits the ground.
During their next session, Eric asks Sarah why there are always people working around the house. Sarah explains the history of her mansion. Her husband William was a noted gun manufacturer, and his rifles caused the deaths of many. William died of tuberculosis while their infant daughter Annie died after just six weeks. Sarah believes her house is haunted by the ghosts of those who died from a Winchester firearm. She has rooms built around the mansion to be designed as the same rooms in which the victims died, and she seals the doors with 13 nails to keep them from haunting the rest of the mansion. Subsequently, Eric goes around asking workers if they have seen anything unusual or ghostly, but nobody gives him a straight answer.
Sarah asks Eric about the bullet he carries with him. He explains that he was shot when his wife went mad before she turned the gun on herself. Eric died for three minutes before being revived. However, he blames himself for what happened to Ruby.
Henry continues to show signs of possession. Eric finds himself talking to someone believed to be a servant of the mansion named Benjamin Block (Eamon Farren), who says the Winchesters must die. Eric goes to talk to Sarah about this, and she shows him a picture of Ben in a confederate uniform. The story is that Ben and his two brothers were soldiers, and the brothers were killed by union officers carrying Winchester rifles. Ben held the Winchesters accountable for the deaths of his brothers, and he shot and killed 15 employees before the police arrived and shot Ben to death. Sarah had the room in which he died recreated in the mansion, which has allowed Ben's spirit to walk among the earth.
An earthquake strikes the mansion in the middle of the hauntings. Some of the workers are killed as the mansion starts to come down. Ben's spirit attacks Sarah and slams her against the walls. Eric sees himself on the night that Ruby died. He tries to take the rifle from her as she is about to kill herself, but she accidentally shoots him in the chest. In her guilt and despair, she shot herself in the head. Ruby's spirit then appears before Eric to tell him to let go of his guilt so that she can be rid of her own.
Ben's spirit is joined by other Winchester victims. Ben attempts to attack the family before Eric shows up to intervene. Somehow, Eric is able to shoot Ben's spirit and put an end to the ghosts' terror.
Eric leaves the Winchester mansion feeling absolved of his guilt, while Henry is better now.