In 1944, Father Lankester Merrin, a parish priest in Holland, was forced by retreating Nazi soldiers to pass life-and-death judgement upon his parishioners. The experience left him with a broken spirit, his faith destroyed.
It is three years later, and Merrin has since abandoned the priesthood. Now working as an archaeologist, he has unearthed a startling discovery in the Turkana district of British East Africa: an ancient Byzantine church, preserved in pristine condition - as if it had been buried on the day it was completed.
The interior walls are covered with vivid mosaics dramatically illustrating the Archangels Michael and Lucifer clashing in battle during the War in Heaven.
This remarkable discovery attracts the attention of the archdiocese in Nairobi, who dispatches a young priest, Father Francis, to Derati to watch over Merrin's work and make sure that the religious aspects of the dig are given the proper consideration. Although he is shocked and troubled by Merrin's lack of faith, Francis devotes himself to setting up a church school for the children of the village.
When Merrin finds a local boy, Cheche, beaten and terribly injured, he brings him to Rachel Lesno, a doctor who runs the Turkana medical clinic. Rachel sets out to help Cheche recover, although his condition is unpromising.
As the dig continues, Merrin discovers that the imposing statues within the church are guarding a secret. Buried beneath the church is an ancient crypt that contains the remnants of satanic ritual and brutal sacrifice. The church was never meant to be used for worship - it was built atop the crypt to contain the evil within it, then immediately buried.
The Derati natives are convinced that uncovering the church has unleashed a demon, and begin to violently clash with the hostile British military troops that have been sent to guard the site.
As the village rapidly disintegrates into madness, Cheche's condition radically and implausibly improves. Father Francis is convinced that Cheche is possessed and must be baptised, but Merrin resists the young priest's pleas that he intervene. However, plagued by deeply disturbing dreams and devastated by the brutality unfolding around him, he realises that he must try to recover his lost faith if they are to have any hope of overcoming the encroaching evil.
The gruelling exorcism that ensues will transport Merrin back to the horrors of the Dutch village where he first saw the true face of evil, and plunge the Derati valley into chaos and death.