The story centers on Father Lankester Merrin, whose profound crisis of faith emerges following the traumatic experiences he endured during World War II. Long before the events depicted in The Exorcist, the youthful Merrin, portrayed by Stellan Skarsgård, embarks on a journey to East Africa as part of a sabbatical from the Church. During this time, he immerses himself in history and archaeology, struggling to mend his fragmented beliefs. Haunted by a horrific episode from his past involving a vicious Nazi SS commander who coerced him into executing innocent lives to save a village, Merrin is burdened by guilt and anguish.
After the war, Merrin is now an archaeologist in Cairo when a collector of antiquities persuades him to join a British excavation in Kenya's Turkana region. This dig site is uncovering a Christian Byzantine church believed to be built around 500 A.D., predating Christianity's arrival in Africa. The collector's request is urgent: Merrin must retrieve an ancient relic of a demon before the British team can lay their hands on it. Teaming up with Father Francis, a Vatican scholar diverted from missionary endeavors, Merrin travels to the excavation site, where they encounter Major Granville, the British officer leading the dig, and a brutish chief excavator with an unsettling appearance.
Upon arrival, Merrin meets Sarah, a physician with a troubled past rooted in her time spent in a concentration camp during the war. Merrin’s guide, Chuma, provides assistance as they investigate why local laborers are disappearing in fear of the church's supposed curse. Alarmingly, Merrin witnesses a digger suffer an unexpected seizure, hinting at dark forces at play.
Exploring the site reveals an almost pristine condition of the church, with an unsettling detail: the angel statues brandish their weapons downward, contrasting with the traditional representation of angels. Merrin and Francis surmise that this suggests the angels are holding back something sinister buried beneath. An even more disturbing sight is the upside-down crucifix hanging in desecration—the sign of a greater evil.
Merrin’s quest for knowledge leads him to consult with the lead archaeologist, Monsieur Bession, only to learn he has succumbed to madness and been transferred to a mental facility. When Merrin confronts Bession, he is met with chilling revelations of demonic possession and ultimately witnesses Bession take his own life. Doubts linger for Merrin about the authenticity of the possession claims, but he reluctantly accepts a volume on Roman rituals for exorcism from Father Gionetti, despite his vow never to perform one again.
As unsettling events escalate, tragedy strikes when a local boy is brutally killed by predatory hyenas. His brother Joseph, witnessing the horror, retreats into a catatonic state. Merrin becomes increasingly convinced that an evil entity resides within the church, which appears to be corrupting the surrounding area. Investigating the tales of a plague that obliterated a nearby village, Merrin is startled to find an empty grave, pointing to deeper malevolence.
Father Francis shares crucial insights about the church's hidden history, revealing it was constructed upon the very site where Satan was cast down post-war in heaven. As they delve deeper, Merrin uncovers an even more ancient temple beneath the church associated with dark rites and demonic symbolism. This harrowing discovery leads him to confront Pazuzu, the demon that he will face again in The Exorcist.
In a climactic turn, it becomes evident that Sarah is the one possessed, culminating in a desperate exorcism within the tunnels beneath the church that tragically claims her life. Merrin, alongside young Joseph, emerges from the now-sand-buried church, echoing a historical cycle of devastation driven by the church's malevolence. As British soldiers and local tribes annihilate each other, Merrin returns to Rome, reconvening with the collector in a café. Explaining his failure to recover the relic, Merrin is met with the enigmatic remark: "But you found something....Didn't you?" Ultimately, he is revealed to have assumed his priestly collar again, signifying his restored faith after triumphing over the demon through the holy rites of exorcism.
