If none of my men volunteer I will accept this challenge!
No King Arthur, I can do this. I am not as valuable to this court, let me do it.
The cahllenge is that I will give you a chance to chop my head off, but I get to do it to you in a year.
I am off to fulfill the challenge.
King Arthur and his men were enjoying a New Year's party when the Green Knight stops in unexpectadly and challenges one of the King's brave knights to a challenge. However none of them respond and it is awkwardly quiet.
This Girdle is magical and it will protect you from death!
A flustered King Arthur volunteers, because everyone else stays quiet. However, Sir Gawain then volunteers and convinces King Arthur that he is not valuable to court, and his service won't be missed, so he should go instead. He accepts the challenge and chops off the Knight's head. The Knight somehow is still able to talk and picks up his head and tells Sir Gawain to come find him in a year so he can chop his head off and keep the deal.
Just do it already, stop hesitating!
Almost a year has past and SIr Gawain heads out to go and find the Green Knight to fulfil his part of the challenge.
I am really the lord from the castle you were at! I struck you because you were not loyal because you kept the girdle
Upon his arrival he was greated with great hospitality. Sir Gawain was supposed to give back to the lord whatever he was given during his stay at the palace, and likewise with the lord. He was given a green girdle by the lords wife, however he kept it and didn't return it like he was supposed to, even though he kept the deal on the other ends. When the time came a knight escorted him the the Green Chapel, where the Knight was.
A knight guided Sir Gawain to the just outside the green chapel where the Green Knight would be. The Green Knight is pleased that Sir Gawain fulfilled the promise. He attempts twice but doesn't make contact. On the third try he cuts Sir Gawains neck a little bit.
The Green Knight explains who he is. He says how Sir Gawain was a loyal knight except in the fact he kept the green girdle, and that is why he struck him in the neck, to punish him for that. Sir Gawain was quite ashamed and embarrassed and felt bad for what he had done.