The Green Knight shows up to Camelot and challenges the Leader to a contest in which one person can strike him with an ax if he can return the blow.
Arthur accepts, but Gawain steps in instead and cuts off the Green Knights head.
The Green Knight picks up his head as if nothing happened and tells Gawain to go to the Green Chapel in a year from now.
A year later, Gawain sets off to the Green Chapel and comes across a kingdom where he stays for 3 days, and makes a deal with the king saying they will trade what they get that day.
After those 3 days, Gawain goes to the Green Chapel and meets the Green Knight and says he is ready, and the Green Knight barely strikes Gawain not killing him, saying he is an honest man and a true knight.