The Fisher King is, in addition to being a film by Terry Gilliam, one of the legends associated with King Arthur. Though there are several versions of the story, the general idea is this:

A young man vows that he will find the Holy Grail. After doing so, he sees a vision of the Grail in the firelight. Overcome by the feeling of power, he reaches out to grasp it, but it vanishes, leaving his hand to be burned in the fire.

Many years later, he has become a king, and has sent many knights to search for the Grail, but he has never found it. He lies in his bed, a broken old man. A jester comes to comfort him, and seeing not a king but a human being in need of help, asks what he can do. The king says that he is thirsty, and would like some water. The jester fetches a cup of water for the king.

After drinking, the king sees that the cup is, in fact, the Holy Grail. He asks, "How is it that you have been able to find what my knights were unable to find?" The jester answers simply, "I do not know. I only knew that you were thirsty."