Lanval is one of the lais of Marie De France.
This one takes place in good ole King Arthur's court and follows the knight Lanval who is facing some dire financial straits. See, Lanval is the (presumably younger) son of a foreign king who came to be a knight in Arthur's court. He's pretty good at it, and all the other knights love him. . . when they remember him, that is.
Apparently Lanval is a very unassuming character and despite being so kind and generous, he is easily forgotten. It gets to the point that- you know how knights were supposed to make their money, right? How they'd go out adventuring/waging war/plundering/pillaging, and they'd bring back all the loot they'd acquired to the king, and then the king would divvy it up among them? Yeah, Arthur just plumb forgets Lanval even exists when the loot-divvying is going on, and I guess Lanval is too damn polite to mention it.
So one day poor sad Lanval goes out for a ride in the countryside and he just so happens to run into the camp of the queen of faeries. Not just the camp, but he also goes inside one of the tents and finds the queen herself lounging half-dressed in bed. Instead of being angry that some snot-nosed human has barged into her place, the queen tells Lanval not only will she be his girlfriend (having fallen in love with him instantaneously, as you do), but she'll also give him all the money he wants and needs. The only rule that absolutely positively cannot be broken is that he cannot tell anyone about her at all, ever. Lanval is totally okay with this.
So after a long stay involving massive amounts of food, brand spankin' new clothes, and lots of faerie ladies running around being pretty, Lanval heads back to the place he's staying and everybody who sees him wonders where the heck the party was. Over the course of the next few weeks, Lanval becomes the most well-liked guy in Camelot. He pays for everybody's everything. He covers tabs at bars, he gives money to the homeless, he bails out all the people in the jails, he donates to charities- he basically becomes Santa Claus. Even the knights remember mid-adventure that- oh yeah, Lanval! And start inviting him along for their games and parties and stuff. For a while, everything is turning up Lanval.
Then Queen Guinevere (Note: in the original poem, the queen married to Arthur isn't named. But you totally know it's Gwen) gets a bee in her bonnet and follows Lanval as he goes off to meet his girlfriend (one of the rules of the two meeting being that he had to be away from people so nobody would see her). Gwen decides that, you know this Lanval fellow isn't so bad, and decides that she wants to be his girlfriend. Lanval politely declines. Gwen threatens to tell Arthur Lanval was trying to hit on her if he doesn't comply. Lanval still says no. Gwen throws a hissy-fit, hurling every insult she can think of at him up to and including questioning his sexuality. Lanval finally loses his cool and says that not only does he have a girlfriend, but she is way prettier than Gwen.
Gwen runs off and sulks in her bedroom for a while before Arthur notices and asks her what's wrong. She accuses Lanval of trying to get with her and then insulting her when she told him no. Arthur is righteously pissed off and has Lanval arrested. The other knights quickly go to Lanval's defense, with all of them putting up their lands and titles as collateral to ensure that Lanval won't try leaving the city or what have you. There is going to be a trial judged by all the local barons (as soon as they all get to town) and then Lanval will be sentenced.
The only way for Lanval to get out of this is if his girlfriend actually shows up and is in fact prettier than Guinevere. All the other knights are hopeful this could happen, but Lanval is thoroughly depressed because he knows he broke the rule and now whenever he calls for the faerie queen, she ignores him.
The day of the trial arrives and Lanval testifies. He swears that he did not try to put the moves of Gwen, and everybody believes him. He does admit to insulting her, though, but says it's not really an insult if it's true. Before Gwen can come down from the stand and throttle him, two beautiful faerie ladies on horses trot on by, distracting everybody in the court. The knights all elbow Lanval and ask him which one of them is his lady love, because either of them are pretty enough to trump Gwen. He says neither and goes back to being sad. The faerie ladies go up to Arthur and say their Lady will be coming by soon, and ask if they can have a room set up. He says yes, and they go off.
Arthur tries to get the barons to deliver a verdict, but they're all too distracted by the pretty faerie ladies to do any logical thinking and ask for a break. Arthur begrudgingly gives them one.
The next day the trial happens again, and this time two more, even prettier faerie ladies ride by. They tell Arthur that their Lady is on her way. Again, the barons need a break. Again, Arthur gives them one, probably regretting not just having Lanval killed without trial.
Finally, on the third time the trial is held, Lanval's girlfriend arrives in all her ethereal queenly splendor. Everybody in the whole damn city comes to watch her ride by on her horse. Lanval is thrilled. The queen tells Arthur that she is totally Lanval's girlfriend. All the barons agree hands down that she is way prettier than Gwen (who is at this point probably frothing at the mouth) and the story ends with Lanval jumping onto the back of his lady love's horse and riding off with her to Avalon.
A translation by Judith P. Shoaf is available online in PDF format for anybody with the urge to read the poem.