Arn Magnusson remembers, as a fearful crusading templar, his youth in fractured Sweden's West Gotland. He and his brother Erskin were friends with king Erik's son Knut, until a neighboring warlord killed the king to seize the crown during the local church's solemn consecration. After Arn's seemingly fatal fall and 'miraculous survival', his mother insisted on keeping a promise to God to offer the boy as oblate to the regional abbey. There abbot Henri allowed father Guilbert, a former templar, to add warrior skills to his monastic education. After an incident in which he was forced to kill a bride-robber in a duel, Arn tries to prevent his family's feud with a haughty neighbor's getting bloodily out of hand.

Arn's brothers wold be-fiancée fails to seduce Arn before he finds out she's the sister of his beloved. Prince Knut, now an adult, is back from Norwegian exile to reclaim the throne. He lures crack archer Arn to help him kill the usurper, but Arn's family fears that only leads to civil war. Arn's siren convinces the bishop that her alleged lover Arn and his true love, her sister, committed mortal sins, so they are confined to strict convents for 'half their lives', 20 years. Arn's abbot however gets his sentence commuted into joining the Knights Templars, a crusading order.