The poem opens right in the thick of things (in medias res is the official term), with an unnamed woman standing before a court of "lords" to offer testimony. The title of the poem tips us off that this is probably Guenevere (you might know her as Guinevere), though she isn't actually named until line 141. If you know anything about the stories of King Arthur (either from kids books or Monty Python and the Holy Grail), you probably know that his wife, Queen Guinevere, cheated on him with Lancelot. But this poem makes us question whether she really did cheat on Arthur.
The poem is mostly a first-person account by Guenevere of her own life and the history of her relationship with Launcelot (a.k.a. Lancelot). Guenevere is apparently on trial for adultery. Gauwaine (a.k.a. Gawain), one of Arthur's knights, seems to be the chief prosecutor of the trial.
Guenevere gains confidence as she goes along, describing not only her actions, but also her emotions, with vivid and colorful detail. She keeps repeating that Gauwaine's accusation (which we never actually hear) is a "lie," but the best evidence she can muster is her own beauty. At the end, the narrator breaks in to say that Launcelot is coming "at good need," presumably to rescue Guenevere.