One New Year’s Day King Arthur and his knights are gathered in the Hall at Camelot, feasting and celebrating. In the midst of the festivities, the door flies open and into the hall rides a huge man upon a huge horse. His face, though fierce, is fair and open, but - wonder of wonders - he is green all over: his clothes, his skin and hair, his horse, even his great axe. He carries a holly bough above his head, which he flings down, and looking around issues a challenge. He has heard of the bravery of Arthur and his knights and although he doubts anyone will have the temerity to step up, he offers to allow such a man to strike him with his axe with the proviso that the challenger must meet him in a year and a day at the Green Chapel so that he can return the blow.
The knights stare at each other in dismay and only Sir Gawain takes up the challenge. He steps forward and strikes so hard that the man’s head rolls off onto the floor. To everyone’s amazement however, the giant man picks up his own head by its green hair, leaps back onto his horse, and after reminding Gawain to meet him at the appointed time and place, he gallops out of the hall, his head swinging in his hand.
The year passes all too quickly and at Michaelmas Sir Gawain sets out on his quest, carrying his shield with its five pointed star, and riding far away through wild lands full of forests, where he fights with robbers and wolves. It is midwinter and bitterly cold. Finally on Christmas Eve, weary and cold to the bone, he comes to a fine castle set in parkland, surrounded by a moat, and with thick battlements and white pinnacles. Here, he is welcomed by the lord, Bertilak, a friendly host, who tells him he may stay as long as he pleases.
The lord introduces Sir Gawain to his beautiful wife, and they spend the three days of Christmas in feasting, dancing and merriment. When Gawain explains his mission, the lord tells him the Green Chapel is only a two hour ride from the castle and Gawain is welcome to remain as his guest until New Year’s Day.
On the fourth day Bertilak announces that he is going hunting and tells Gawain to remain at the castle with his wife in order to rest and prepare for his ordeal. He then strikes a bargain with his guest: he will present Gawain with whatever he hunts down in the woods, and in exchange Gawain is to give him anything that may come his way in the castle. Gawain agrees.
When his host has left, Gawain remains in his bed, sleeping deeply, until all at once he is woken by the lady of the castle. She comes to sit on his bed, and at once attempts to seduce him, but in spite of her best efforts and his unwillingness to offend her, he yields nothing but a single kiss. When Bertilak returns and gives Gawain the deer he has killed, his guest responds by returning the lady's kiss to Bertilak, but doesn’t divulge its source. The following day the lady comes again, and obtains two kisses - but no more - from the steadfast Gawain, and there is a similar exchange of a hunted boar for the kisses. On the third morning, she offers Gawain a gold ring as a keepsake, and when he gently refuses she pleads that he at least take her girdle of green and gold silk which is charmed and will keep him from all physical harm. This is too difficult to resist and Gawain accepts it, whereupon they kiss three times. That evening, Bertilak returns with a fox, which he exchanges with Gawain for the three kisses - but Sir Gawain keeps the Girdle.
Then it is time for him to keep his appointment. With a heavy heart he armours himself, taking up his lance and his shield of red with its gold pentangle, and mounting Gringolet his fine steed. Bidding his hosts farewell he leaves the castle.
Soon he arrives in a desolate valley beneath a cliff where he finds a green mound on a level space beside a stream, hollow as a cave inside. Echoing around the hills is a sound as of an axe being whetted on a stone, and out of the hollow comes the Green Knight himself, crying that it is time for Gawain to keep his promise.
Gawain bends his head to receive the stroke but flinches at the sound of the blade whistling through the air. The Knight lowers his axe scornfully. Gawain swears to take the next stroke without flinching, which he does - and the Knight stays his hand yet again. The third time he strikes he cuts only the skin of Gawain’s neck and then lowers his axe once more. “You have born the blow bravely,” he says, “and I will strike you no more. The first and second blows that did not strike you were for the promises you kept for my wife’s kisses in the castle, which you truly returned to me. But you failed the third time, giving me the three kisses but not the green girdle, and so I wounded you. But you hid the girdle for love of your life, which is a little sin, and so I pardon you.” Then he laughs merrily and Gawain recognises him as Bertilak the lord of the castle
They shake hands, and Gawain rides home to Camelot, still carrying the green girdle in memory of his ordeal.