Sir Gawain and the Green Knight is a late 14th century alliterative romance recorded in a single manuscript, which also contains three other pieces of an altogether more Christian orientation. The four poems are linked by their use of a North or West Midland dialect of Middle English. The core of the story of Sir Gawain and the Green Knight is far older and embraces many elements central to Celtic mythology, the most prominent being the "severed head" theme, though it is also coloured by events of the time, chief amongst which was the Black Death.

The manuscript, Cotton Nero A.x, is in the British Museum. The first modern edition was published by J. R. R. Tolkien and E. V. Gordon in 1925.

Sir Gawain and the Green Knight is written in the style that linguists have termed the Alliterative Revival of the fourteenth century. Instead of focusing on a metrical syllabic count and rhyme, the alliterative form relied on the agreement of (usually a pair of) stressed syllables at the beginning of the line with (usually) a third and fourth at the end of the line. The line always finds a "breath-point" at some point after the first two stresses, dividing the line into two half-lines, separated by the pause called a caesura. Although the Gawain-poet was somewhat freer with convention than his predecessors, this more or less had been the form of alliterative poetry going back into the Old English. The Gawain-poet, however, did embellish the form with some end-rhyme, as it happens. His structure has come to be known as the bob and wheel. The poet broke his alliterative lines into variable-length groups and ended these nominal stanzas with a rhyming section of five lines known as the bob and wheel: one one-stress line rhyming a (the bob) and four three-stress lines rhyming baba (the wheel). These lines also alliterated. (See the next section for an example.)

The Summary of the story

The Challenge
The story begins at King Arthur's court at Camelot on New Year's day. As Arthur's court is feasting, a stranger, the gigantic Green Knight, mounted on horseback and armed with an axe (but ambiguously he also carries a spray of holly, suggestive of peace), enters the hall and lays down a challenge. One of Arthur's knights may take the axe and strike a single blow against the Green Knight, on the condition that the Green Knight will return the blow one year and one day later. Sir Gawain, the youngest of Arthur's knights, diffidently accepts the challenge and chops off the giant's head. The Green Knight, still alive, picks up his own head, reminds Gawain to meet him at the Green Chapel in a year and a day, and rides off.

Sir Gawain's journey
Almost a year later, on the day after All Hallows Day, Sir Gawain sets off in his finest armour, on his horse Gringolet, to find the Green Chapel and complete his bargain with the Green Knight. His shield is marked with the pentangle, which the poem attributes to Solomon [Stanzas 27-28], and which is to remind him of his knightly obligations. The journey takes him from the isle of Anglesey to a castle somewhere in the West Midlands, where he arrives on Christmas Eve. Gawain meets the lord of the castle and his beautiful wife, who are pleased to have such a renowned guest. After the feasting of Christmas Day, the lord inquires why Gawain has journeyed so far from home during the holiday season. Gawain tells of his New Year's Day appointment at the Green Chapel and that he must continue his search the next day. The lord laughs and insists Gawain must prolong his visit, for his search has ended: the Green Chapel is not two miles away! [ll. 1068-78]

The lord's bargain
That night, the lord announces that while he spends the next day hunting, the travel-weary Gawain shall stay at the castle, sleep as late as he wants (even through Mass), and eat whenever he chooses to arise; the lady will keep him company. But to add a little interest to the day, the lord proposes a bargain: he will give Gawain whatever he catches, on condition that Gawain gives to the lord, without explanation, whatever he might gain during the day. Gawain accepts. The next morning, after the lord has gone, the lady of the castle visits Gawain's room and tries to seduce him, claiming that she knows of the reputation of Arthur's knights as great lovers. Gawain, however, keeps to his promise to remain chaste until his mission to the Green Chapel is complete, and yields nothing but a single kiss. When the lord returns with the deer he has killed, he hands it straight to Sir Gawain, as agreed, and Gawain responds by returning the lady's kiss to the lord. According to the lord's bargain, Gawain refuses to explain where he won the kiss.

On the second morning, Gawain again receives a visit from the lady, and again politely refuses her advances. That evening, when the lord returns, there is a similar exchange of a hunted boar for two kisses. On the third morning, when the lady visits his chamber, Gawain maintains his chastity but accepts a green silk girdle, which is supposed to keep him from harm, as a parting gift. But, the lady insists, he must not tell her husband. That evening, the lord returns with a fox, which he exchanges with Gawain for three kisses. However, Gawain keeps the girdle from the lord so that he can use it in his forthcoming encounter with the Green Knight, thus violating their agreement.

The meeting with the Green Knight
The next day, Gawain leaves for the Green Chapel, with the lady's silk girdle hidden under his armor, and accompanied by a guide from the lord's castle. Leaving the guide, who is afraid to approach the Green Chapel, Gawain finds the Green Knight busy whetting the blade of an axe in readiness for the fight. As arranged, the Green Knight moves to behead Gawain, but only strikes him on the third axe-swing, the blow barely cutting his neck and only injuring him slightly. The Green Knight then reveals himself to be an alter ego of the lord of the castle, Bertilak de Hautdesert, and explains that the three axe blows were for the three occasions when Gawain was visited by the lady. The third blow, which drew blood, was a punishment for Gawain's acceptance of the silk girdle. There is much speculation as to whether the girdle would have really kept Gawain from dying had the Green Knight desired to kill him. The lady, it seems, has lied to Gawain insofar as the girdle has not kept him completely from harm. On the other hand, it has kept him from death. The author leaves the exact powers of the girdle undefined and open to interpretation, but makes it clear that the Green Knight would not have willingly spared Gawain's life had he failed to resist the lady's sexual advances. Assuming it has no life-saving powers, it is meant to be ironic that the girdle, the one thing that Gawain thinks will save him, is actually the thing that harms him; furthermore, assuming the girdle has no real powers, it would have been the thing that led to his death had he taken it as a love token, which is what the lady originally offered it to him as.

The Green Knight explains that Gawain's trial was arranged by "Morgne the goddes", Morgan le Fay, mistress of the wizard Merlin and now a guest at Hautdesert castle. A passage of rhetorical anti-feminism follows which has excited considerable discussion in the critical literature, where Gawain blames his troubles on women in general. Arthur refuses to blame Gawain and decrees that all his knights should henceforth wear a green sash in recognition of Gawain's courage and honor and to recognize the fallability of men. The two men part on cordial terms, Gawain returning to Camelot. There, Sir Gawain recounts his adventure to Arthur and explains his shame at having partially succumbed to the lady's attempts, if only in his mind. The poem concludes with the motto: "HONY SOYT QUI MAL PENCE", which is a form of 'honi soit qui mal y pense', which is the motto of the Order of the Garter. From this, it has been concluded that Gawain's peers wearing the sash is meant to represent the origin of the Order of the Garter.