The story is set in 1209, in a remote monastery on the west coast of Ireland, where a group of monks is tasked with safeguarding a sacred relic—a stone said to have been used in the martyrdom of Saint Matthias. The relic is believed to possess immense power, and the monks consider it vital to their faith.

Tom Holland plays Brother Diarmuid, a young and naive novice who is dedicated to the monastery and has never known life beyond its borders. Jon Bernthal portrays The Mute, a mysterious and seemingly haunted man who has taken a vow of silence and serves the monastery as a protector. Richard Armitage plays Raymond de Merville, a ruthless Norman nobleman who becomes involved in the mission.

The monks are ordered by a Cistercian emissary, Brother Geraldus (played by Stanley Weber), to transport the relic to Rome, where it is to be used to secure support for a new Crusade. Reluctantly, the monks agree to the journey, and they embark on a dangerous pilgrimage across Ireland’s rugged landscape. The journey is fraught with peril, as the land is torn by tribal warfare, and their path is fraught with both physical and spiritual challenges.

Brother Diarmuid is thrust into a world far more brutal than anything he has known. The journey quickly turns deadly, as the monks are attacked by Irish clansmen and other hostile forces, all of whom seek to possess the sacred relic for their own purposes. The Mute, who accompanies the monks as their protector, proves to be a formidable warrior, using his skills to defend the group against constant threats.

As they travel further, it becomes evident that Brother Geraldus is willing to go to any lengths to protect the relic, driven by a fanatic belief in its power and significance. The group’s mission is complicated further when they encounter Raymond de Merville, who offers his protection but has ulterior motives. Raymond is motivated by power and sees the relic as a means to increase his influence.