I. Textual History of the Myth
Lost original probably composed mid 12th-c. by French poet. Tale of immediate enormous popularity, retold several times by contemporary poets. But no complete version has survived.
Among the Tristan romances composed in Old French in the 12th century (see also Rougemont, p. 26 n. 1):
1) Longest fragment is that of BÈroul (ca. 1165?), "vulgar" version;
2) Nine fragments of Thomas's ("courtly") version of the legend (ca. 1173-76); Thomas composed his romance for the Anglo-Norman court of Henry II and Eleanor (of Aquitaine);
3) A lost romance composed by Chrétien de Troyes ("Iseut and Marc").
II. Bédier's reconstitution of the legend
Summary of romance as reconstituted in the 19th century by Joseph Bédier (see also Rougemont, pp. 26-30):
Tristan is son of Blanchefleur (sister of King Mark of Cornwall) + Rivalen, King of Loonois. "Tristan" (="sad"), because sadness of his birth, which caused the death of his mother Blanchefleur. Trained by Governal, then at the service of his uncle King Mark. Martial deed: Tristan kills a predator threatening Cornwall (demands tribute of young men and women) a giant, named Morholt, whose sister is Queen of Ireland. Tristan is wounded, seeks cure, sails alone to Ireland. Meets a jongleur who teaches him the art of singing poetry. Is cured by Isolde (Yseut), the daughter of Queen of Ireland. Returns to Cornwall.
Mark under pressure by his barons to marry. Wants the lady possessing blond hair, one of which brought to Cornwall by a swallow. Tristan seeks that woman, whom he identifies as Isolde. Both sail, accompanied by Brangain, Isolde's lady-in-waiting. But drink a love potion prepared by Queen for Isolde and Mark. Indissoluble love.
Marriage of Mark and Isolde. Brangain as substitute for wedding night. Secret love denounced by Mark's barons. Tristan banished from court. Occasional secret meetings. Discovered and denounced by dwarf, sentenced to death. But Tristan escapes, and rescues Isolde. Live in forest of Morrois. But discovered by Mark's forester. Mark goes to forest alone, replaces Tristan's sword by his. Clemency that inspires Isolde to return to Mark (in Béroul: because effect of love potion disappears after three years).
Tristan offers his service to Hoel of Brittany. Marries his daughter, Isolde aux Blanches Mains (Isolde 2). But cannot forget Isolde ("the blond"), does not consummate marriage. Isolde 2 tells her brother Kaherdin that she still a virgin. Kaherdin and Tristan go to Isolde 1: secret meetings, where Tristan disguised as leper, penitent, and madman.
Back to Brittany, Tristan fatally wounded in combat: only Isolde 1 can save him. Sends her a messenger: if Isolde 1 is in the boat, white sail. But Isolde 2 lies and tells him that sail is black. Tristan dies of grief; Isolde 1 arrives, and dies at his side. Buried side by side, two trees grow on their grave with entwined branches.
III. Marie de France: "The Honeysuckle"
Of "France," but living at the Anglo-Norman court of Eleanor and Henry II, for whom she composed her Lais (verse short-stories), circa 1160.
IV. Denis de Rougemont
Born in Switzerland, a right-wing philosopher; famous for his "Love in the Western World" (1939) and his theory of passionate love as generated by obstacles.