Sir Gawain and the Green Knight follows a classic quest formula, with a knight receiving a challenge, going out on a journey to meet that challenge, and returning home to report on his quest.
At Christmas, a knight who is completely green rides into King Arthur's hall. The Green Knight proposes a game: Any knight brave enough to strike off the Green Knight's head may keep the Green Knight's ax, but that man must accept a return stroke in one year. Gawain accepts the challenge and cuts off the Green Knight's head. The knight picks up his severed head and leaves, telling Gawain to look for the Green Chapel.
Near the end of the allotted year, Gawain sets out in search of the Green Chapel. He finds a castle in the wilderness. The lord of the castle asks Gawain to stay until New Year's Day, because the Green Chapel is nearby. The lord proposes an agreement: He will go out hunting while Gawain stays at the castle, and the two men will exchange whatever they have gained at the end of the day.
The exchange of winnings takes place over three days. Each day, the lord goes out hunting, while the lady of the castle tries to seduce Gawain in his bed. Gawain politely refuses her advances, although he does give her some kisses. Finally, she offers him a magic belt that will protect the life of any man who wears it. Gawain repays the lord his lady's kisses, but he does not mention the belt.
Gawain keeps his appointment at the Green Chapel. The Green Knight raises his axe to cut off Gawain's head, but twice he draws back. The third time, the Green Knight barely cuts Gawain on the neck. The Green Knight reveals that he was Gawain's host and that his appearance as the Knight was made possible by Morgan le Fay. He tells Gawain that the first two blows were for the first two days of their agreement, when Gawain fairly repaid him his wife's kisses. The small cut was for accepting the belt and concealing it. Overcome with shame, Gawain acknowledges his fault and wears the belt to remind him of his fault. When he returns to Camelot, the entire court wears green sashes in fellowship with Gawain.