As King Arthur was outside of his castle, he met a mysterious man called the Green Knight. Arthur offers him food, but the knight refuses. Arthur then asks if he is therefor a dual, and the knight says that no one could defeat him. He offers to have someone try and cut his head off with the ax. In return, he has one blow at them.
"If I were hasped in my armor and high on my horse, You haven't a man that could match me, your might is so feeble." (Line 61-62)
Sir Gawain steps up to the knight's offer. The rest agree that his life is less important than King Arthur's. He took the ax and cut the knight's head off. To their surprise, the knight stands up, grabs his head, and rides away.
"'I beseech you, Sire,' he said, 'Let this game be mine.'" (Line 118-119)
Gawain then sets out to finish his pact. He has to find the knight at the Green Chapel. He stops at a castle along the way for food and shelter. While he is there, the lady of the house attempts to seduce him and give him gifts. He refuses, except on the last day, he took a green sash.
"'If taking my ring would be wrong on account of its worth, And being so much in my debt would be bothersome to you, I'll give you merely this sash that's of slighter value.'" (Line 200-202)
Sir Gawain reaches the Chapel (a rugged place) to finish his pact. The Green Knight is there sharpening his blade, and he is ready to take Gawain's head off.
"'You can't be Gawain,' he said, 'who's thought so good, A man who's never been daunted on hill or dale!'" (Lines 252-253)
The knight swings the ax, but Gawain's head doesn't fall. As soon as Gawain notices that he isn't dead, he jumps up with his helmet and grabs his sword, ready to fight.
"'I owed you a stroke, and I've struck; consider yourself Well paid. And now I release you from all further duties." (Lines 320-321)
Gawain finds out that the lord from his trip was the knight, and he was sending his wife to test Gawain. He passed except for when he took the sash. After admitting to his sin, Gawain is cleansed by the knight. They part ways once they agree on a truce.
"Whatever harm I've had, I hold it amended Since now you're confessed so clean, acknowledging sins And bearing the plain penance of my point; I consider you polished as white and as perfectly clean As if you had never fallen since first you were born.'" (Lines 367-371)