Shortly after arriving in the new city of Constantinople in the fourth century AD, Tontorix and Diaecus are enticed into the Roman army for the secret purpose of converting military officers from the cult of Mithras to Christianity. Assigned to serve in present-day Cologne, Germany, they come into direct and perilous conflict with the Ripuarian Franks and Alemanni along the border of the Rhine River and must decide how far they will go to achieve their objective.

Hundreds of years later, from the seventh to the ninth centuries, the two ageless friends travel along trade routes east into China, where they encounter martial arts and live through Emperor Wuzong's persecution of Buddhism.

In the twelfth century, after returning to the west and hearing about the crusade for the Holy Land, they travel to Jerusalem to visit the most famous religious sites in the world. Their involvement in the siege of Ashkelon leads them into Egypt where the Kingdom of Jerusalem, the Zengid dynasty, and the Fatimid caliphate battle for control over the strategically important land and its people.

Ultimately, after centuries of observing different faiths, spiritual practices, and religious conflicts throughout the world, they come to a shared conclusion about divine existence.