On his way to the Sword Bridge into Gore, Lancelot encountered a most beautiful lady who offered him hospitality for the night on condition that he sleep with her. Agonizing over it, but seeing no other way to pursue his quest - presumably because he needed shelter for the night - he promised to do as she asked. Her castle was spacious and well appointed, but she and Lancelot seemed to be intirely alone in it, with neither squire nor servant. This in itself signals strangeness in a milieu where privacy was not only rare but seldom sought for its own sake by anyone except the religious.
Lancelot saw no one else at all until, dutifully going to her bedrom, he found her apparently about to be raped by one rough knight while two more knights and four men-at-arms, stood by. Lancelot wreaked havoc upon them until the lady called them off, for they were in fact her servants and the whole thing had been staged. She and Lancelot then lay down together in a fine bed in the middle of the hall, but he kept his shirt on and she her chemise. At last, understanding his unease, she took pity and left him alone in bed.
Next day she asked to go with him for some distance, if he would escort her acording to an old custom of Logres that any knight who molested an unescorted damsel was held in disgrace everywhere forever, but if she were escorted, he had only to defeat her escort in order to do as he wished with her. Knowing his own worth, Lancelot promised to keep her safe.
On their way, they found a gold and ivory comb which "Portia" somehow recognized as Guenevere's. Overjoyed, Lancelot took the golden hairs from it and kept them as a cherished treasure, calmly giving the comb to "Portia" with never a thought that his lady the queen might have thanked him for its return. Farther along, they met the "Young Knight of the Meadow", who had long loved "Portia" and announced his delight at finding her under circumstances that would allow him to take her.
To get better ground for fighting, however, they went on to a meadow, where the "Young Knight's" father sat on a Spanish sorrel watching his people at backgammon, chess, dice games, rounds and jigs, singing, tumbling, leaping, and wrestling - not just "frivolous" pastimes, as the author assures us.
This "Old Knight of the Meadow" talked his son into postponing the combat until they had followed the lady and strange knight long enough to take his measure. Arriving at the "Church of the Tombs" and learning from its monk that Lancelot had just moved a marble slab that should have needed at least seven strongmen to budge it, thus proving his identity as the man who would free the foreign prisoners of Gore, the "Knights of the Meadow" prudently returned home.
For her part, "Portia", piqued because Lancelot would not tell her his name, only that he came from King Arthur's realm, quitted him soon after they left the church.
Chrétien's translater and commentator D.D.R. Owen remarks in a note that the episode defies any logical explanation. To me it looks crystalline: I could wish everything in the romance to be equally obvious, if I didn't suspect the work of being comedy in various levels.
The neighboring knight's attentions have so wearied this lady that in order to rid herself from him she is ready to sell her body to some other champion. The apparent absence of any other people in her castle is meant to lend credence to the danger of her being raped in her own bedroom. The staged assault is a test of Lancelot's prowess. Seeing him not only superb at fighting, but ready to protect her even without his own sexual involvement, makes her willing to waive the "you must sleep with me" condition. When she asks to accompany him under the peculiar custom described above, she hopes that her unwanted suitor will try to take her away and end up decisively trouned by her new escort. The pique which finally causes her leave Lancelot probaby owes as much to the fact that this battle never occurs as to his refusal to tell her his name.
It remains possible if not probable that, in some lost source of Chrétien's, the original version of this episode constituted a further test of Lancelot's worthiness to enter the "land from which no stranger returns". For this reason, and because her castle lies near the marches of Gore, "Portia" seems to me as a suitable name for this lady.