An epic fantasy adventure based on the timeless Arthurian legend, The Green Knight tells the story of Sir Gawain, King Arthur's reckless and headstrong nephew, who embarks on a daring quest to confront the eponymous Green Knight, a gigantic emerald-skinned stranger and tester of men.

A young Viking prince strives to become a knight in King Arthur's Court and restore his exiled father to his rightful throne.

A NASA spacecraft proves Einstein right when, traveling faster than light, it ends up near King Arthur's Camelot. On board are big-hearted Tom Trimble and Hermes, the look-alike robot he built. Tom immediately makes friends with pretty Alisande while becoming enemies with the evil knight Sir Mordred. It seems Mordred has joined up with the Sorcerer Merlin and they are both up to no good. It is now up to Tom to try and use 20th century technology to foil their plans.

Gawain was a squire in King Arthur's court when the Green Knight burst in and offered to play a game with a brave knight. Gawain journeys across the land, learning about life, saving damsels, and solving the Green Knight's riddle.

The film chronicles Perceval's knighthood, maturation and eventual peerage amongst the Knights of the Round Table, and also contains brief episodes from the story of Gawain and the crucifixion of Christ.

One Christmas, a knight garbed all in green appears in King Arthur's Court and challenges any knight to hew off his head on the proviso that he be allowed to return the axe stroke afterwards. All of King Arthur's other knights being frightened, Sir Gawain takes up the challenge and hacks off the green knight's head. The green knight then gives Gawain a year to find his home and live a little before the cut is returned.