The Green Knight comes to Camelot hall, proposing a Christmas game. This game says that any one individual in the hall can strike the Green Knight with one blow of his ax. However, in a year, the favor will have to be repaid.
Sir Gawain volunteers to behead the Green Knight in the place of King Arthur. He partially does this to prove his worth, and partially because it seems like an unfit trade off for a king to partake in. Sir Gawain then beheads the Green Knight.
After the beheading, the Green Knight, the Knight picks up his head and mounts his horse. He reminds Gawain of his necessary fulfillment at the same time next year,
At the end of the next year draws near, Sir Gawain goes to fulfill his promise. He meets a lord named Bercilak who is able to direct him to the Green Chapel, where he can find the Green Knight. Lord Bercilak allows Gawain to stay at his castle.
For two days, while the lord is away, the lady of the castle tries to woo Gawain. He rejects her advances, only accepting a kiss each day, which he exchanges with the lord for his hunting game.
The lady offered him a small gift to remember her by and they settled on her green girdle. She promises him that when he is wearing it, no hero on earth can harm him.