Guillaume de Launfal was the very model of a knight errant – the placard boy for Toussaint, if you will. When Geralt first met the young man, he was valiantly tilting at a windmill... which, to be fair, turned out to be hiding a ferocious giant.
Getting himself into trouble clearly counted among Guillaume's favorite hobbies. The second time Geralt met him, he had to rescue the knight from an enraged shaelmaar trying its best to rip him to shreds in a packed arena.
Wounded yet grateful, Guillaume asked the witcher to meet with him about a very urgent matter.
The matter turned out, as it matters' wont in Toussaint, to be an affair of the heart. Guillaume was tormented by unrequited love for the duchess' lady-in-waiting, Vivienne de Tabris. He suspected her frigidness and distance must be the result of some malicious curse. Geralt, a professional curse-breaker, naturally took the contract. To get a chance at an audience with Lady Vivienne, he had to take the wounded Guillaume's place in the knights' tourney.
The witcher determined Vivienne was indeed afflicted by a curse and had been since before she was born, a dark malediction which caused her to transform into a bird every night. Geralt offered to try to cure this cumbersome condition.
If Geralt tells Guillaume details about the curse:
He reported back to Guillaume on what he had learned. Guillaume rose to the occasion and offered to take the curse upon himself in order to free Vivienne from it.
Guillaume convinced Vivienne to undergo a ritual which would transfer her curse to him. According to Geralt, once transferred the curse's effects would be weakened, but he was not able to make any guarantees concerning the young knight's fate.
As soon as they had performed the ritual, Vivienne felt the curse had been lifted. As for Guillaume, he did not notice any significant side effects.
If Geralt checks in with the couple later:
Guillaume and Vivienne's story had a happy ending. Geralt ran into the couple in Beauclair. It turned out Guillaume had grown a few feathers on certain parts of his body. Vivienne considered them rather flattering, so all was well.
If Geralt does not tell Guillaume details about the curse:
Vivienne asked him not to reveal her secret to anyone, so Geralt did not tell Guillaume about his plans.
Geralt performed a ritual freeing Vivienne from the curse. She decided to travel abroad in order to enjoy the life she had left to the utmost.
Guillaume felt like he had been rejected and drowned his sorrows in wine. Geralt tried to explain to him one cannot force love on anyone.
If the player finishes The Warble of a Smitten Knight without Guillaume's help, he can be found drunk in Beauclair before The Pheasantry Inn after the main storyline is finished. He will blame Geralt for not having Vivienne and he will then fall asleep on the ground.