King Arthur and his Knights are feasting in celebration of New Year's. A Green Knight comes in and challenges one of his knights to blow for blow, which will be delivered in a year and a day's time.
Gawain accepts and then beheads the Green Knight. The Knight then picked up his head, told Gawain to meet him in the Green Chapel and then left.
Gawain ventures out in search of the Green Chapel and it takes him nearly all year.
Gawain comes across this great castle and makes a bargain with its king that everything the king hunts, he gives to him while he gives the king everything he finds.
The Queen of the castle tries to tempt Gawain and kisses him so Gawain kisses the King. This happened for two days. On the third day, she begs him to take her sash as a token. He eventually relents and doesn't give it to the king. He heads out for the Green Chapel.
The Green Knight makes him wait then comes down. Gawain flinches the first time and is mocked. The Knight then doesn't swing hard, then he swings and nicks Gawain. He knew all along and in fact was the King. The King doesn't fault Gawain for loving his life. he was testing his honor. Gawain returns to Arthur alive and well.