[Backstory: Tristan, nephew of King Mark of Cornwall, and Yseut, King Mark's wife, accidentally drink a magic potion, which compels them to fall passionately in love and have an affair under the King's nose. Enemies of the queen accuse her of adultery, and to acquit herself, Yseut must swear a solemn oath on holy relics, before all of King Mark's court and in the presence of King Arthur himself, on a sacred hill, just at the edge of a muddy marshland.]
The time was approaching when the queen was to exculpate herself. Tristan, her lover, had no qualms about disguising himself in motley clothes. He wore woolen garments and no shirt. He wore a rough woolen tunic and his shoes were patched. He had had made a wide cloak of coarse wool, which was blackened with smoke. He had disguised himself extremely well and looked more like a leper than anything else.
Dinas was with the queen and saw what was going on. He winked at Tristan, who he well knew was under the cloak. Dinas addressed the queen.
"My lady," he said, "your fine cloak is going to be quite spoiled. This marshy ground is full of slime. I am worried that you may get some of it on your clothes."
Yseut was not afraid, and she smiled at him and winked. Then Dinas realized what the queen had in mind. She turned and spoke to the leper:
"I want you to do something for me."
"Kind and noble queen, I come without hesitating but I don't know what you could mean."
"I do not want to get my clothes muddy. You will be my donkey and carry me carefully across. Turn your face that way and your back this way. I will get on like a boy."
Then the leper smiled and bent his back. The queen mounted. The fair Yseut rode on his back with her legs round him. The leper kept his face down as he arrived on the other side. Yseut let herself slide off.
Everyone sat down in rows. Gawain stood near the relics and the household of Arthur was seated round. Arthur began to speak:
"Listen to me, fair Yseut, and hear what you are accused of. You must swear that Tristan never loved you wickedly or wrongfully, and only bore you the love he owed to his uncle and his wife."
"My lords," she said, "by the mercy of God I see holy relics here before me. Listen now to what I swear, and may it reassure the king: so help me God and St. Hilary, and by these relics, this holy place, the relics that are not here and all the relics there are in the world, I swear that no man ever came between my thighs except the leper who carried me on his back across the ford and my husband, King Mark. Those two I exclude from my oath; I exclude no one else in the world. From two men I cannot exculpate myself: the leper and King Mark my lord. If anyone wants me to do more, I am ready here and now."
All those who hear her oath could bear no more.
"God," said everyone, "that would be a cruel wrong! She has done everything that justice demands, and she put more into her oath than the villains required her to. A curse on anyone who mistrusts her now!"
–– Béroul, The Romance of Tristan, Translated by Alan S. Fedrick. London: Penguin, 1970 (excerpt edited and abridged by Bard)