The narrative begins at Camelot during New Year festivities, where King Arthur refuses to eat until he witnesses a marvelous sight. A gigantic knight, entirely clad in green and riding a green horse, bursts into the hall carrying an enormous axe and a holly bough. He proposes a chilling game: any courtier may strike him with his own axe on the condition that he receives a return blow in a year and a day. Sir Gawain steps forward, accepts the challenge, and neatly lops off the Green Knight's head. Miraculously, the headless figure picks up his own head, reiterates the terms of the compact, and rides away, reminding Gawain to seek him at the Green Chapel by the next New Year.
As autumn turns to winter, Gawain departs from Camelot in full armor, carrying a pentangle shield that symbolizes truth and perfection. He endures a lonely and perilous journey through remote wildernesses, battling harsh weather, wild beasts, and giants until he reaches a magnificent castle on Christmas Eve. The lord of the castle warmly welcomes him, and Gawain learns that the Green Chapel lies only a couple of miles away. The host proposes a pact: the lord will hunt in the woods each day while Gawain rests at the castle, and at night they will exchange whatever winnings each has acquired.
During the next three days, while the lord hunts deer, a boar, and a fox, the lord's wife pays stealthy visits to Gawain's bedchamber. She attempts to seduce the knight with eloquent words and flattering affection. Gawain maintains strict courtesy and honor, resisting her advances while accepting only kisses, which he dutifully transfers to his host each evening in fulfillment of their agreement.
On the third day, however, the lady presses him harder and offers him a ring, which Gawain refuses. She then offers a green silk girdle, claiming it possesses magical properties that make the wearer invincible to any mortal blow. Tempted by the prospect of surviving his impending encounter at the Green Chapel, Gawain breaks his rule, accepts the girdle, and vows to keep it a secret from her husband. That evening, he kisses the host thrice but intentionally conceals the gift of the green lace.
On New Year's morning, Gawain rides out to find the Green Chapel, wrapping the magical girdle twice around his waist. He discovers a wild, eerie hollow and meets the Green Knight, who brandishes his great axe and prepares to strike. Gawain flinches slightly at the first feint of the blow, earning a mild rebuke for cowardice. When the knight swings a second time, Gawain stands as still as stone, but the blade is again withheld. On the third swing, the axe nicks his neck slightly, drawing a single drop of blood onto the snow.
Furious, Gawain leaps back, draws his sword, and declares the terms of the covenant fulfilled. The Green Knight rests on his axe and laughs, revealing that he is actually Bernlak de Hautdesert, the lord of the castle. He explains that the two harmless swings corresponded to the two days Gawain honestly returned the kisses, while the nick on the third day was punishment for concealing the green girdle. The entire trial was orchestrated by Morgan la Fay to test the renowned knights of the Round Table and frighten Queen Guinevere.
Overwhelmed by shame and self-reproach for his lapse in honesty and his surrender to cowardice, Gawain confesses his failure and renounces the girdle as a token of cowardice and covetousness. The Green Knight forgives him completely, declaring him as pure as if he had never sinned, but Gawain insists on wearing the green lace thereafter as a permanent reminder of his human frailty. Returning to Camelot, Gawain recounts his adventures honestly to Arthur and the court, who comfort him and resolve that all knights of the Round Table shall wear a green band in his honor.
