The book is a modern translation of a 14th century alliterative poem, which tells the story of Sir Gawain, a knight of King Arthur's Round Table. The story begins when a mysterious Green Knight arrives at Arthur's court, challenging the knights to a beheading game. Gawain accepts the challenge and beheads the Green Knight, who surprisingly survives and tells Gawain to seek him out a year later to return the blow. The book follows Gawain's journey, filled with trials and temptations, as he tries to fulfill his promise to the Green Knight, maintaining his honor and chivalry.