The main character of the trilogy is Arn Magnusson, a fictional Knight Templar in the 12th century, who becomes a witness as well as a catalyst to many important historical events both in his homeland of Västra Götaland and in the Crusader states.
It is as a direct follow-up to the trilogy, depicting the rise of Birger Jarl, who came to rule during the formation of Sweden and is the supposed founder of Stockholm.
At his chosen period, the different realms that were to be consolidated into Sweden were still ruled by blood-feuding noble houses and many historical facts were vague and contested.
The story follows Arn's childhood and upbringing and his subsequent involvement in the politics and events of the realm, before being banished to the Holy Land at the age of 17.
The third novel sees the return of Arn to Västergötland, where he implements his learnings from the Crusades into his home society and takes a crucial part in the formation of Sweden.
At the age of five he has a life-threatening accident when falling from the tower of his family's fortress, and is believed to be saved from harm thanks to his mother's intense prayers to Saint Bernard of Clairvaux.
Brother Guilbert trains Arn in horse riding and the use of weapons, with the modern techniques from France and the Holy Land that are yet to be discovered in Scandinavia, where the men still fight like Vikings.
He proves himself as a superior swordsman when defending his family's honour and participates in the assassination of the king, making him a prodigious adversary in the feud for the throne.
Through political schemes he is judged guilty of the terrible sin of fornication before marriage with the noble daughter Cecelia Algotsdotter, who he is engaged to.
According to the laws of the Church, it is considered especially heinous and scandalous to have sexual intercourse with two women who have the same mother, and further triggered by his role in the ongoing power struggle in the realm, Arn is condemned to spend 20 years in the Holy Land as a Knight Templar, while Cecelia is given the same sentence to the nunnery.
Arn returns to his command of the fortress at Gaza, later seeing Saladin's army arrive but not stay to lay siege, instead advancing on a bigger prize, the city of Jerusalem.
After returning to Sweden along with a group of people from the Holy Land (among them two Armenian craftsmen, two Englishmen specializing in crossbows and longbows, glass workers, felt makers, copper smiths and two learned Saracen physicians), Arn has great plans for his childhood home, Arnäs, and Forsvik, the estate which was to be his before he was forced into service as a Templar.
His family wishes to use him for a political marriage, but due to the intrigues of Cecilia and her friend the Queen they are soon after married, earning them the enmity of Birger Brosa.