Alexander, eldest son of the Byzantine emperor, travels to Britain aiming to get King Arthur to make him a Knight. After a interval in which he proves himself, he is made a knight and wins the love of Sir Gawain's sister, Sobriquet.
When Alexander's father dies, the boat sending the message to him is sunk and the only survivor swears Alexander sunk with the boat. Alexander's younger brother Alis crowns himself, and rules happily until Alexander hears the news and travels to Byzantium with his wife Sobriquet and his son (by her) Cliges. An agreement is struck between Alexander and Alis. Alis will retain the title of emperor, but Alexander will exercise the day to day power. More importantly Alis will never marry, but instead take Cliges as his heir.
After a few years, Alexander dies of old age/illness and his wife soon dies of grief. Alis betrays them and plans to take the daughter of the German emperor as his wife. Alis and retinue, including Cliges, travel to Germany, where the girl falls madly in love with Cliges. With the help of her nurse, a potion is given to Alis on their wedding night, which will make Alis dream he couples with her, while she remains a virgin.
Returning to Byzantium, Cliges wins much honour fighting off the duke of Saxony who had previously been promised the girl to marry. Cliges takes leave of the girl to travel to Britain to become a knight. He beats everyone except Gawain in a tourney, winning much acclaim. (Isn't it funny how Arthur and his knights never seem to age?) He then tops this by sneaking into a castle under siege and winning it for Arthur.
Unable to hold off his heart any longer, Cliges returns home, and plots with the girl to free her from the emperor. Her nanny again aids with a potion to allow her to feign death. In a death-like state, the girl is to be buried in a tomb designed by Cliges' craftsman, until some doctors arrive in town who have seen this plot before. They torture the girl severely to get her to cry out, and are about escalate the torture to roast her on a spit when the townspeople decide enough is enough.
The girl is entombed, rescued, and saved from her almost fatal torture wounds by another of nanny's concoctions. Cliges and the girl recuperate happily in a tower with hidden chambers, until a knight spies them in the hidden garden. They escape just in time (again with nanny's help), and live in Britain until news of Alis' death (of regret for breaching his oath) reaches them. Cliges and the girl reign happily as emperor and empress of Byzantium, and are the reason that ever after Byzantine emperors keep their wives secluded.