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.

Arn is now a seasoned Templar, enjoying trust and respect of the grand master. His chivalrous behavior towards Muslim passersby wins him the respect of sultan Saladin, who was anonymously among them. When Saladin's army marches to besiege Jerusalem, he sends a retainer to offer Arn safe passage, which is declined. Arn correctly analyzes the enemy's best route and sets a masterly trap, which forces Saladin to retreat for the time being. Back home, Arn's beloved Cecilia gave birth to their son, which was taken away, and suffers a horror regime under the Sverker abbess. She befriends another Cecilia, who turns out the fiancée of Knut. When he comes collect her, the abbess must promise a decent treatment.

At the end of his penance term of 20 years, Templar Arn expects to return home with an honorable discharge from the late grandmaster after a last desert mission, during which he must defeat fellow knights to prevent Bedouin allies being slaughtered. The grandmaster's successor tears it up, claiming victory over Saladin is at hand and imposing foolish risks. In Sweden, his bride Cecilia is offered to succeed the wicked abbess, who dies cursing her for refusing to forgive. However she chooses to leave, visit their son Magnus, now a man, and seeking to find and marry Arn at last.