The Romance of Tristan and Iseult by Joseph Bédier - also known as Tristan and Isolde and other names, is a medieval chivalric romance told in numerous variations since the 12th century. Based on a Celtic legend and possibly other sources, the tale is a tragedy about the illicit love between the Cornish knight Tristan and the Irish princess Iseult. It depicts Tristan's mission to escort Iseult from Ireland to marry his uncle, King Mark of Cornwall. On the journey, Tristan and Iseult ingest a love potion, instigating a forbidden love affair between them.
The story and character of Tristan vary between versions. His name also varies, although Tristan is the most common modern spelling. The earliest tradition comes from the French romances of Thomas of Britain and Béroul, two poets from the second half of the 12th century. Later traditions come from the vast Prose Tristan (c._1240), markedly different from the tales of Thomas and Béroul.
After defeating the Irish knight Morholt, the young prince Tristan travels to Ireland to bring back the fair Iseult (often known as Isolde, Isolt, or Yseult) for his uncle King Mark of Cornwall to marry. Along the way, Tristan and Iseult ingest a love potion which causes them to fall madly in love. The potion's effects last a lifetime in the legend's courtly branch. However, in the common branch version, the potion's results end after three years.
In some versions, Tristan and Iseult ingest the potion accidentally. In others, the potion's maker gives it to Iseult to share with Mark, but she gives it to Tristan instead. Although Iseult marries Mark, the spell forces her and Tristan to seek each other as lovers. The King's advisors repeatedly try to charge the pair with adultery, but the lovers use trickery to preserve their façade of innocence. In Béroul's version, the love potion eventually wears off, but the two lovers continue their adulterous relationship.
Like the Arthur-Lancelot-Guinevere love triangle in the medieval courtly love motif, Tristan, King Mark, and Iseult all love one another. Tristan honors and respects his uncle King Mark as his mentor and adopted father. Iseult is grateful for Mark's kindness to her. Mark loves Tristan as his son and Iseult as a wife. However, every night each has horrible dreams about the future. Simultaneous to the love triangle is the endangerment of a fragile kingdom and the end of the war between Ireland and Cornwall (Dumnonia).
King Mark eventually learns of the affair and seeks to entrap his nephew and wife. Mark acquires what seems to be proof of their guilt and resolves to punish Tristan by hanging and Iseult by burning at the stake. However, Mark changes his mind about Iseult and lodges her in a leper colony. Tristan escapes on his way to the gallows, making a miraculous leap from a chapel to rescue Iseult.