Three men - Deacon, a skeptical religious brother, Gray, an English layman and technology expert, and the stringent Father Mark Amidon - are sent by the Vatican to investigate reports of supernatural activity in an old, recently reopened, thirteenth century church located in the Devon countryside.Upon their first visit to the church, local priest Father Crellick tells them a miracle has taken place, while Gray sets up recording equipment. Crellick shows them footage of objects on the altar mysteriously moving but Deacon remains skeptical. The next day, Mark discovers a hidden side panel but he is disturbed by an unseen force before he can enter. A multi-mic radio setup detects the sound of deep growls and whispers, followed by the sound of an infant crying. Mark pursues a despondent Crellick up the bell tower to the roof; Crellick questions whether he has witnessed a miracle or something far worse - he leaps to his death before a horrified Mark.The inhabitants of the village start to behave with hostility towards the team. Local youths burn a sheep to death outside the men's cottage. The local pub landlord evicts Deacon and Gray from his premises after overhearing the two men discussing local folklore and the credibility of the tenets of old pagan religions versus those of Christianity. Deacon continues to investigate and traces whispers and creaks to a hidden door, inscribed with a pagan vigil he has seen in the diaries of the last minister to serve at the church until it was closed in the 1880s. Behind the door, a set of stairs leads down into darkness, and Deacon is assailed by the sounds of a baby crying and Crellick screaming.Deacon brings in his elderly mentor from the Vatican, exorcist Father Calvino, in an attempt to purify the church grounds. Calvino explains that during the founding of Christianity in England, priests built churches upon the sites of pagan temples. He has evidence that the church is situated on a site - still visible in aerial photographs - of human sacrifices to an unnamed pagan deity.That night, during the exorcism, violent and invisible forces shake the church. Mark is apparently killed and Calvino's eyes bleed before their bodies mysteriously vanish. Deacon traces distant cries to the hidden staircase. As Gray and Deacon descend into a subterranean labyrinth, they find evidence of child sacrifice and realize that the former minister had converted to worshiping the pagan deity. Seemingly spotting Mark, who walks into the darkness heedless of their calls, the pair find Calvino's ornamental crucifix on the floor of one of the tunnels. Following the sound of Mark's voice, they crawl through a narrow, foul-smelling passageway whose exits suddenly contract via a membranous material before the tunnel itself begins to move. The walls begin to secrete a powerful digestive enzyme which horribly burns the two men. As their lights go out, Gray screams in agony and terror while a tormented Deacon recites the Lord's Prayer.