The film opens with the execution of a woman accused of witchcraft in 1845 in Banfield, Massachusetts. She is hanged from a tree and ultimately set on fire. Before she dies, however, her spirit is bound to the body of a doll.

Many years later, disgraced journalist Gerry Fenn works a job reporting on all things strange and unusual. His latest assignment takes him to Boston, where the purported strange activity is revealed to be a teenage prank. As he's preparing to leave, he discovers the doll, shown earlier in the movie, and purposely crushes it, accidentally setting the woman's spirit free. Gerry later gets into an accident as he's driving away after seeing a young girl, Alice, run across the road. He follows her to the tree from the prologue, the site where he found the doll, and hears her speak before collapsing. When he takes her to a nearby church, he learns that Alice is a deaf-mute and couldn't have spoken. Gerry decides to stay in town to investigate what he thinks could be a real story rather than a hoax or prank.

The following day Alice stuns the community by speaking and proclaiming that she has been cured by the Virgin Mary. This launches a media furor that is further propelled by other seemingly miraculous healings. The Catholic Church sends Monsignor Delgarde to investigate the claims, assisted by Bishop Gyles. Gerry befriends Alice, during which time he learns that while she has been speaking to a being named Mary, the girl has only assumed that this is the religious figure. Beginning to suspect that something sinister is occurring due to visions of "Mary" as a horrific entity, Gerry finds a partner in Father Hagan, Alice's uncle, who was healed by Alice but also suspects the healings' true nature. Father Hagan discovers papers detailing the occurrences, but before he had the chance to warn anybody he is attacked by Mary and his body is found hanging in the church. Bishop Gyles convinces Gerry to keep the apparent suicide of Hagan quiet as it will tarnish the divine occurrences. His death is declared an accident.

Gerry befriends Natalie, the town's physician, and reveals that he was found fabricating stories, the reason he is now found reporting hoax stories but hoping this event will relaunch his career. The pair eventually comes across the information Father Hagan discovered, revealing the death of Mary Elnor, the woman from the prologue. Mary sold her soul to Satan in order to gain eternal life and power. He would allow her and her descendants, revealed to be Alice, to perform "miracles" so people would pledge themselves to her and in turn, Satan. To the shock of Gerry and Monsignor Delgarde, they learn that Bishop Gyles was aware of the entire situation but chose to keep it secret since the miracles have attracted people back to the church. He also warns Gerry that any attempt to tell the truth will fall on deaf ears due to his tarnished reputation.

Alice, who they believe is unaware of Mary's true nature, wishes to hold a service by the tree and broadcast it to the masses. Delgarde warns that this is a satanic ritual. Anyone who pledges allegiance to Mary will condemn their soul to Hell. Delgarde, along with Gerry and Natalie, tries to perform a ritual in the church to stop this. However, Mary appears and kills him, crushing him with a burning cross.

Once the service begins, Alice urges everyone to pledge themselves to Mary three times. Gerry manages to stop the crowd from fully pledging by claiming that the miracles were just more of his hoaxes, that they were all a result of the placebo effect. Natalie communicates with Alice using sign language, urging her to stop the service. Mary speaks to Alice, entreating her to continue the ritual or never speak again. Realizing Mary's true nature, Alice confirms that the miracles were false. The tree suddenly bursts into flames, causing all the attendants to flee in panic as an angry Mary emerges from it. She incinerates Bishop Gyles before trying to kill Gerry. She is stopped by Alice, who sacrifices herself to save his life. This in turn causes Mary to disappear, as Alice was her link to the living world. Struck by her sacrifice, Gerry begs God to save Alice's life. She comes back to life but is once again a deaf-mute. All the people whom Alice cured soon relapse. In the epilogue, the statue of Mary inside the Church cries tears of blood.

https://en.wikipedia.org/wiki/The_Unholy_(2021_film)
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