The vanished wall reveals an eldritch forest where summer and winter, sun and moon, mingle impossibly. The knights marvel as a Green Knight—a figure of twigs and foliage clad in ancient mossy mail—rides into the hall on a horse made of roots and bracken. He declares that whoever defeats him will learn his secret, and the knights eagerly compete for the challenge.

Sir Villiars wins the right to fight. Wielding a two-hander with skill, he initially dominates, landing hard blows on the seemingly slow Green Knight. But the Green Knight has been feigning sluggishness; he suddenly accelerates, shatters Villiars's knee and arm, hammers him to the ground, and stabs him through the neck under his helmet. Villiars dies on the floor, and the knights lay him on the Round Table in grief.

Collum, burning with shame because the marvel came from his own suggestion, insists on fighting next despite Bedivere and Nimue's protests. He fights without full armor, gambling on speed and vision. Recalling Marshal Aucassin's lessons, he trades blows with the Green Knight, who nearly kills him with a blow that knocks off his steel cap.

Desperate, Collum resorts to a dirty schoolyard trick: he traps both blades in a bind, grabs them together with his gloved hand, and twists the Green Knight's sword from his grasp, disarming him. The Green Knight bows in concession. Bedivere quietly congratulates Collum.

The Green Knight removes his helmet, revealing only foliage—no head. A small polecat scampers out from the armor and speaks the secret in the knight's voice: one more knight of the Round Table still lives, and the creature can lead them to him. Bedivere accepts, declaring they leave at dawn.