During a celebration at King Arthur's court, a green knight rides in and offers to submit to one blow on the terms that he would return the blow in a year and a day. Sir Gawain accepts the off and chops the knight's head off. The green knight then picks his head up, reiterates the terms of the bargain and rides off.
The following November Sir Gawain sets of to find the green knight at his magic chapel and fulfill his part of the agreement. During his quest he comes to a castle and meets the lord and his lady. Upon learning the green knight's chapel was nearby Sir Gawain decides to stay there.
The lord and Sir Gawain have an agreement that while Gawain stays there, the lord will go out and hunt while Gawain stays in the castle with the lady and at the end of the day the two men will exchange the fruits of the day.
Over the next three day, the lord goes hunting and Sir Gawain is visited by the lady of the castle who tries to seduce him. Sir Gawain only gets kisses from her. He keeps up his end of the bargain with the lord and exchanges kisses with him for his hunt of the day. On the third day however, the lady gives Gawain a magic girdle that will protect the wearer from death, and he does not tell the lord this.
On the day he is to meet the green knight, Sir Gawain sets out to find him with guide who urges him to give up the quest, saying he won't tell anyone. Sir Gawain refuses and goes to meet the knight, wearing the girdle.
The green knight takes his sing at Sir Gawain who flinches before the ax strikes, which provokes the knight to taunt him saying he must not be as brave as he is said to be. The knight finally strikes him and does not kill him because f the girdle. The knight then reveals that he is actually the lord and the lady of the castle was actually Morgan Le Fay, Gawain's aunt. The knight says Gawain was untruthful for not telling him about the girdle.