I've always wondered about this story, but never actually read it, maybe because I didn't have a copy, or maybe because it always seemed more like a myth than an actual story. As it turns out, it is an actual story and in this translated version, pretty damn readable. 
 The story is simple: King Arthur is having a party with all his knights of the round table (and many more), when suddenly a mysterious Green Knight appears. He isn't all that impressed with all these knights, whom he could easily beat in a proper fight, but he does want to play a little game with them. This game, however, is a strange one; one strike each, to chop off the other's head. The knight who decides to play, can go first. At first, King Arthur is outraged by this, picking up the gauntlet in rage, but then the more sensible knight, Sir Gawain, steps in. He is ready to take on this challenge, if his king will allow it. 
 Great story and I can see how many sword & sorcery writers have gotten some inspiration from this. Highly recommended.