A green knight riding a green horse rides into the hall and wants to take up a challenge for someone is to strike him once with his axe on one condition that the green knight will return the blow in a year and a day. Arthur himself is prepared to accept the challenge but Gawain wanted the honor.
blow for a blow in a year and 1 days time
The green knight bends down and bares his neck before Gawain and he beheads him in one stroke, but the green knight does not fall he picks up his head and shows it to everyone and then rides out of site.
When the date comes Gawain sets off to find the green knight
Gawain comes across a castle where he meets the lord of the castle, and his wife, who are pleased to have guest. Gawain tells them of his challenge with the green knight and that he only has a few days remaining. the man laughs, and tells him that the path that will take him to the green knight and proposes that Gawain rest at the castle till then. Gawain agrees.
The lady take off her belt and offers it to Gawain, a girdle of green and gold silk which, the lady tells him it is a charm that will keep him safe from any physical harm because as he may die the next day, Gawain accepts it.
On the day Gawain binds the belt twice around his waist. He finds the Green Knight, as promised, Gawain bends his bared neck to receive his blow. the green knight goes deliver his blow but only causing only a slight wound on Gawain's neck
This was all a test of Arthur's knights, I was the lord at that castle.