Peace of cake! I'll just kill you and you won't get to swing at me.
You can chop my head now, but in a year I get a swing at you.
Sir Gawain's Journey
At the beginning of the story, King Arthur is having people over in his hall, Sir Gawain being on of them. Then the Green Knight shows up and says he wants to play a game, but it involves the chopping off of heads. Sir Gawain volunteers to play the game because he says that he if he dies, it will not make a big difference.
The Green Knight's Castle
The Green Knight explains the rules of the game. Sir Gawain can take a swing with an ax and try to chop off the Green Knight's head, but in one year Sir Gawain has to come find the Green Knight and the Green Knight gets to do the same thing. Sir Gawan chops off the Green Knight's head, but then the Green Knight picks up his own head and it starts talking.
Green Knight Gets a Swing
Sir Gawain keeps his commitment to the Green Knight and sets out to find him one year later. He takes his horse along with him.
You did not follow my instructions... Keep the girdle as a reminder.
On his way to find the Green Knight, Sir Gawain unknowingly stops at the Green Knight's castle. There he meets the wife of the lord of the castle, and she wants a kiss from him. After they kissed, the lady gave Sir Gawain a green girdle as a gift.
What's mine is yours
Give me a kiss
Sir Gawain makes it to the Green Knight's cave, and the Green Knight attempts to chop off Gawain's head. Gawain keeps flinching, so it takes three attempts. At the end of their encounter, the Green Knight ends up slicing just a part of Sir Gawain's neck, only leaving a little blood.
The Green Knight reveals that he is the lord of the castle, and that his wife is the woman that Sir Gawain kissed. The Green Knight says that he sliced Sir Gawain's neck because he did not fully follow their agreement. Sir Gawain apologizes, and the Green Knight says to keep the green girdle as a reminder of their meetings together.