The libretto is based on an ancient Celtic myth or legend, which was very popular during the Middle Ages.  It was already known in the seventh century, but whether it originally came from Wales or Brittany is a disputed point.  It was very widely known, however, and, thanks to the wandering minstrels, it was translated into all the Continental idioms, and became the theme of many poets, even of later times.  Since the days when Godfried of Strasburgh wrote his version of the story it has been versified by many others, among whom, in our days, are Matthew Arnold and Swinburne.  While the general outline of these various versions remains the same, the legend has undergone many transformations, but Wagner has preserved many of the fundamental ideas of the myth, which is intended to illustrate the overpowering force of passion.  The scene was originally laid in Ireland, Cornwall, and French Brittany.

Blanchefleur, sister of King Mark of Cornwall, falls in love with Rivalin, who dies shortly after their union.  Withdrawing to her husband’s castle in Brittany, Blanchefleur gives birth to a child whom she calls Tristan, as he is the child of sorrow, and, feeling that she cannot live much longer, she intrusts him to the care of her faithful steward, Kurvenal.  When the young hero has reached the age of fifteen, his guardian takes him over to Cornwall, where King Mark not only recognises him as his nephew, but also designates him as his heir.

Tristan has been carefully trained, and is so expert in the use of his arms that he soon excites the envy of the courtiers, who are watching for an opportunity to do him harm.  The King of Cornwall, having been defeated in battle by the King of Ireland, is obliged to pay him a yearly tribute, which is collected by Morold, a huge giant and a relative of the Irish king.  Morold, coming as usual to collect the tribute money, behaves so insolently that Tristan resolves to free the country from thraldom by slaying him.  A challenge is given and accepted, and after a terrible combat, such as the mediaeval poets love to describe with minute care, the giant falls, after wounding Tristan with his poisoned spear.

The King of Cornwall, instead of sending the wonted tribute to Ireland, now forwards Morold’s head, which is piously preserved by Ysolde, the Irish princess, who finds in the wound a fragment of sword by which she hopes to identify the murderer, and avenge her kinsman’s death.

Tristan, finding that the skill of all the Cornwall leeches can give him no relief, decides to go to Ireland and claim the help of Ysolde the princess, who, like her mother, is skilled in the art of healing, and knows the antidote for every poison.  Fearing, however, lest she may seek to avenge Morold’s death, he goes alone, disguised as a harper, and presents himself before her as Tantris, a wandering minstrel.

His precarious condition touches Ysolde’s compassionate heart, and she soon uses all her medical science to accomplish his cure, tenderly nursing him back to health.  While sitting beside him one day, she idly draws his sword from the scabbard, and her sharp eyes perceive that a piece is missing.  Comparing the break in the sword with the fragment in her possession, she is soon convinced that Morold’s murderer is at her mercy, and she is about to slay her helpless foe when an imploring glance allays her wrath.