King Arthur and the Knights of the round table are celebrating Christmastide, when their jolliness is interrupted by a knight clad all in green. He issues the challenge that one man of the court must take one swing at him with his axe and in a year and a day the Green Knight would issue an equal blow back.
After many days of travel Sir Gawain came upon a small castle upon a hilltop. He was welcomed joyously by the Lord of the castle. At dinner that night Sir Gawain's host offered to enliven the proceedings with a pledge. The Lord was going hunting the next day and offered to exchange what he caught with whatever Sir Gawain received in the castle.
The next day Sir Gawain set out to find the green chapel, he was guided by a servant of the castle to the path that led to the Green Knights dwelling. Eventually he came to the entrance of a large intimidating cave, after only a few steps inside the Green Knight appeared and welcomed Sir Gawain.
The Green Knight told him that it was time to uphold his end of the bargain,Sir Gawain complied and continued to get ready to be beheaded. The Green Knight raised his axe, and brought it down. It slightly cut the skin on Gawain's neck. The Knight then then turned into the Lord of the castle and explained to Gawain that the cut was for the green girdle that he did not return on the third night. The Lord let Sir Gawain keep the girdle as a token of his long journey.
Sir Gawain then returned home to Camelot where he was welcomed with great joy. From that day on he always wore the girdle "as a reminder, should he ever get too proud, of his faint heartedness."