This book tells a story of serendipity. Two Christian monks left China about 1274, headed to Jerusalem. Travelling on an itinerary similar to that Marco Polo had taken, they reached Iran, ruled by a Mongol dynasty, the Ilkhans. There, what they never had expected happened: one of them, Mark by name, was elected Patriarch of the Church of the East (with the name Yahballaha), while the other, Rabban Sauma, was sent as ambassador to the pope and the courts of France and England by the Mongol Ilkhan Arghun.

From Rabban Sauma's report of his embassy, and the two monk's memories of their journey from China to Mesopotamia, an anonymous author compiled a biography of Sauma and Mark. He interspersed their report and memories with a narrative about "the occurrences of their time - what happened to them, through them or because of them, relating everything just as it happened".

The result was a chronicle entitled "History of Mar Yahballaha and Rabban Sauma", a rich and lively testimony of a time of unprecedented interconnectedness in the history of Eurasia at the epoch of the Mongol Empire.