The Green Knight shows up at Arthur's castle and challenges somebody to take a swing at him and in return in a year he returns the swing to the opponent.
Arthur steps up, but then Gawain takes his place and takes his swing at the Green Knight and cuts off his head and the Green Knight picks up his head.
A year goes by and Gawain sets off to find the Green Castle. Gawain finds a castle along the way and the King of the castle and him for three days give each other what the receive.
Gawain receives a Green Girdle from the Kings wife and doesn't give it to the king because it is supposed to save him from harm and he sets off for the Green Knight.
Gawain reaches the green castle and the Green Knight starts to take his swing at Gawain and Gawain flinches and the Green Knight gets mad and Gawain says he won't flinch again and the GK takes another swing and Gawain doesn't flinch.
When the Green Knight takes the other swing he only slightly cuts Gawain's neck and reveals he is the King of the Castle and he knows about the Green Girdle and lets Gawain keep the Green Girdle and invites him back next year.