The Green Knight barged into King Arthur's castle with his trusty horse. The Green Knight looked at the knights and announced that he challenges any knight to steal his ax and to lay a blow to him, but in a year's time he will do the same.
Sir Gawain decided to take this challenge and in doing so he cuts off the Green Knight's head. The Green Knight didn't die though, he picked up his head and headed off awaiting for Sir Gawain to return.
The Return Blow
It was nearly a year since the Green Knight challenged him and he is heading off to the Green Chapel. Instead of finding the Green Chapel, Sir Gawain finds a castle that belongs to Sir Bercilak. To stay there at the castle he has to give anything he has been given during the day at the castle in exchange for anything Sir Bercilak has from the chase.
The Green Knight or is he
That morning when Sir Bercilak went hunting, Sir Bercilak's wife went into Sir Gawain's bedroom and gave him a green belt that was supposed to protect him from harm. When the exchange happened Sir Gawain gave Sir Bercilak a kiss on the cheek instead of the green belt that he was given.
Sir Gawain set off to the Green Chapel where he found the green knight sharpening his ax. When the Green Knight hit him with his ax all there was, was a small cut on his neck.
The Green Knight revealed that he was actually Sir Bercilak and that the only reason that he was given a small cut on the neck was because he did not give his wife the green belt and broke his promise. Sir Gawain kept the belt as a sign of the mistakes he had made and was worn by the all knights as a badge of the highest honor and purity of heart.