Casca is in Venice in the 13th Century and saves the life of a young Venetian called Marco who has become involved in a family dispute over a girl. In order to avoid the rival family's henchmen, Marco agrees to accompany his recently returned father and uncle to the Holy Land where the newly elected Pope, Gregory, is to receive his father who has a message from the far distant lands of the Mongol khan, Khubilai. Marco, together with his friends and Casca, set off and land in Acre.
Although the new pope receives them well, and gives Marco's father a reply to send to the khan, Acre is attacked during the night and Casca barely manages to rescue Marco and his family and friends, although one dies bravely fighting the attackers.
Together the small party decide to head east for Hormuz where they hope to get a boat to the east. However the journey is hard and even with Casca's knowledge of the area, they find it difficult to make their way. When they do reach Hormuz, plague is prevalent and Casca's party is chased out of the city and their belongings are lost. That night another of their party succumbs to disease.
Undeterred, the diminished group head north for the Silk Road and cross the Pamir Mountains but winter cuts their journey short and Casca, Marco and his friend Vittorio spent the winter in the mountain temple of Buddhists. When the spring comes they rejoin Marco's father an duncle and make their way into Mongol territory and are taken to the camp of the local warlord who recognises Casca as the "Old Young One" from legend. During the feasting Casca defeats the warlord's son in a wrestling match and is rewarded with a night with his daughter.
They make their way to Khubilai's summer city and are greeted warmly. Casca is greeted by Khubilai and the khan's son, Chen-Chin, befriends Marco and Vittorio. Casca though soon becoems aware of a plot to kill the khan and his supporters by traditionalist Mongols and shortly afterwards he helps in defeating an attempted coup when one of Khubilai's generals, Nayan, defects. Casca escapes an assassination attempt and becomes one of Khubilai's generals in the campaigns to defeat the rebels. During this time Casca meets and falls in love with a young girl called Ko.
After defeating the Burmese, Casca returns to find Chen-Chin dying and realises the young heir to the throne has been poisoned, and subtly forces the traitor, one of Khubilai's trusted aides, to reveal himself. however, the traitor escapes after killing Ko. In the vengeful pursuit, Khubilai's army defeats the rebel army and Nayan is executed while Casca confronts and strangles the man who had killed Ko.
Marco's father and uncle request leave to return to the pope and Marco and Vittorio agree to come with them. Casca agrees to go, too, and they escort a bride for the Mongol khan of Persia part of the way back. Casca returns to Venice and takes his leave of Marco and his family. He wanders north to a village and remains there for a few years until plague takes all the villagers, and then becomes part of the English army at Crecy before moving north to Scandinavia and for a number of years takes up whaling, before returning south again.
Eventually he ends up in southern Spain where he eventually boards a ship bound for the open seas, a boat called the Kuta.