Father Lankester Merrin is a Catholic priest who, having experienced the horrors of WWII, has lost his faith and is seeking answers through his archaeological expeditions. In British-administered East Africa, he meets a doctor named Sarah who is a member of the British military forces stationed there.
Merrin becomes involved in excavating an ancient Byzantine church which, according to legend, was built to contain the remains of an evil entity. The initial signs of something terrible lurking beneath the church are confirmed when several people, including a young boy, are found mutilated near the site.
The local villagers are convinced that uncovering the church has released a demon, and tensions rise between the British military and the natives. The military commander, Major Granville, is determined to keep order by any means necessary, even if it means violently suppressing the local population.
As the violence escalates, Merrin finds himself drawn into a battle against the demon that has taken possession of Sarah. He must confront his own inner demons and ultimately make a choice between his faith and his duty to protect innocent lives.
The movie explores themes of faith, redemption, and the consequences of colonialism on indigenous cultures. It is a prequel to the original 1973 film The Exorcist, which also features Father Lankester Merrin as a central character.