The Green Knight bursts into Arthur's court at Camelot on New Year's Eve, riding a horse that is also entirely green. The poet describes him in extraordinary detail: he is "half-giant" in stature, with green skin, green hair, a green beard, and green clothing trimmed with green fur. He wears no armour, rides barefoot despite the winter season, and carries a holly branch in one hand - symbol of peace - and a massive green axe in the other. His eyes blaze red, and his overall appearance is both terrifying and strangely beautiful.

The challenge he proposes is the "beheading game": any knight may strike him one blow with his axe, on the condition that the challenger must seek him out in a year and a day to receive the same blow in return. When no knight immediately accepts, Arthur himself rises to the challenge, but the young Sir Gawain steps forward and asks to take the king's place. Gawain beheads the Green Knight with a single stroke - but the supernatural figure calmly stands, retrieves his severed head, which continues to speak, reminds Gawain to meet him at the Green Chapel in a year, and rides away.

The beheading game motif has precedents in earlier Celtic and French literature, including the Middle Irish Bricriu's Feast (c. 1100) and the French Le Livre de Caradoc, but the Pearl Poet's treatment is unique in its depth and moral complexity. The Green Knight's ability to survive decapitation and his green colouration connect him to the "Green Man" figure found carved into churches across Britain - a face made of leaves symbolizing the rebirth of vegetation each spring.

When Gawain arrives at Bertilak's castle after a long and perilous journey, the lord proposes an exchange-of-winnings game: Bertilak will give Gawain whatever he catches while hunting, and Gawain must give Bertilak whatever he "catches" in the castle. Over three days, Bertilak's wife attempts to seduce Gawain. On the first two days, Gawain honourably exchanges kisses with Bertilak as agreed. On the third day, however, the lady gives him a green girdle that she claims will protect him from any harm. Fearing for his life in the forthcoming encounter with the Green Knight, Gawain accepts the girdle but does not surrender it to Bertilak that evening, breaking their agreement.

At the Green Chapel, the Green Knight prepares to behead Gawain. He strikes twice, stopping short each time - testing Gawain's courage. The third strike nicks Gawain's neck, drawing a small line of blood. The Green Knight then reveals himself as Bertilak and explains the meaning of the three blows: the first two were for the first two days when Gawain faithfully exchanged his winnings, while the nick was punishment for failing to surrender the girdle. Gawain returns to Camelot wearing the green girdle as a mark of his shame - though the other knights adopt it as a symbol of honour, establishing the Order of the Green Garter.

The poem was composed during the reign of Richard II (1377-1399), a period of political uncertainty that scholars have connected to the poem's preoccupation with honour and moral testing. The poet's identity remains unknown, though the same manuscript contains three other poems - Pearl, Patience, and Cleanness - which share the poet's distinctive alliterative verse style and sophisticated theological concerns.