Bertilak first enters the poem in his enchanted appearance as the Green Knight. At Camelot on New Year's Day, he carries a great axe and a holly branch into King Arthur's hall. Rather than asking for ordinary combat, he offers a beheading game: a knight may strike him once, provided that the challenger will seek him out after a year and a day to receive a return blow. Gawain accepts and severs his head. The stranger nevertheless retrieves it, mounts his horse, and reminds Gawain to keep his promise at the Green Chapel. His departure leaves the knight bound to an encounter that appears certain to end in death.

Near the following Christmas, Gawain finds shelter at a castle after travelling through North Wales and the Wirral. Its welcoming lord is Bertilak, though his name and connection to the Green Knight are not yet revealed to the guest. The household includes Bertilak's wife, commonly called Lady Bertilak, and an elderly woman later identified as Morgan le Fay. Bertilak assures Gawain that the Green Chapel is nearby and persuades him to rest. He then proposes an exchange of winnings: whatever he gains while hunting will belong to Gawain, and whatever Gawain receives in the castle must be given to him in return.

The exchange unfolds over three days. Bertilak hunts deer, then a boar, then a fox, while his wife tests Gawain's courtesy and loyalty in the bedchamber. On the first evening Gawain passes on one kiss in return for the venison; on the second he gives two kisses for the boar. On the third day the lady offers a green girdle that she claims will protect its wearer from being killed. Fearful of the approaching axe stroke, Gawain accepts it and agrees to keep it secret. That evening he gives Bertilak three kisses but conceals the girdle, breaking the exchange agreement.

At the Green Chapel, the Green Knight raises his axe three times. Gawain flinches at the first attempt but holds still at the second, which is also withheld; the third gives him a slight wound on the neck. The knight explains that the first two harmless feints correspond to the days when Gawain kept their agreement, while the cut answers his concealment of the girdle. He knows about the lady's visits because he arranged the test himself. He then names himself Bertilak de Hautdesert and explains Morgan le Fay's role in his transformation. Morgan sent him to challenge the Round Table's reputation and frighten Guinevere. Bertilak forgives Gawain, judging his failure less harshly because it arose from a desire to live.

The revelation makes Bertilak's hospitality part of the same trial as the beheading game. His hunts and his wife's visits lead back to the agreement made in Arthur's hall, testing whether Gawain will keep his word when his life seems at stake. Gawain retains the girdle as a reminder of his fault, while Bertilak invites him back to celebrate with the household. The name appears in editions and translations as Bertilak, Bercilak or Bernlak de Hautdesert. Bredbeddle, the green challenger in the related poem The Greene Knight, belongs to a different retelling rather than an additional episode of this same biography.