The movie is a prequel to the classic horror film "The Exorcist" and follows the character of Father Merrin before his encounter with the possessed Regan MacNeil. Father Merrin is shown as a frail and troubled man, haunted by his experiences during World War II and questioning his faith. As he works on the archaeological dig, he begins to uncover ancient symbols and artifacts that suggest the presence of evil forces in the area.
Meanwhile, a young boy named Cheche who lives in the nearby village is suffering from a mysterious illness that seems to be getting worse. Father Merrin comes to suspect that Cheche may be possessed by a demon and begins to perform exorcisms on him, trying to drive out the evil spirit. Along the way, he enlists the help of a local doctor named Rachel who has her own hidden past.
As the exorcism ritual becomes more intense, Father Merrin must confront not only the demon within Cheche but also his own personal demons from the past. The film culminates in a dramatic confrontation between the forces of good and evil, with Father Merrin facing a final, terrifying choice.