Sir Gawain and the Green Knight (c.1380)

There's actually a pretty terrible take on this poem starring Sean Connery as the Green Knight himself - I only know this because I actually took a class on Arthurian literature and watched it there (hah). But again, if it helps the plot stick...

Characters: King Arthur, Gawain (his nephew), The Green Knight (Lord Bertilak de Hautdesert), Lady Bertilak, Morgan le Fay, Gringelot (Gawain's horse)

Plot summary:

King Arthur is having one of his feasts. 

A completely green knight barges into the hall. Sets a challenge. The challenge is that he will allow any one of Arthur's knights to chop his head off, but if he survives, the knight who struck the blow must be beheaded in turn, one year and one day later.

Gawain, Arthur's nephew, accepts the challenge. He beheads the Green Knight, but the Green Knight just picks up his head and puts it back onto his shoulders. 

One year later, Gawain sets off to find the Green Knight. He stops in at a lord's castle which he finds out is near the Green Chapel, where the Green Knight is to be found. 

Gawain stays at the lord's castle for 3 days, during all 3 of which the lord's wife tries to seduce him while the lord is out hunting. 

On the 3rd day, the lord's wife gives Gawain a girdle which has magical powers - whoever wears it won't be hurt. Gawain was supposed to give the girdle to the lord of the castle, but he doesn't, because he figures it'll be useful for him when he meets the Green Knight.

The next day, Gawain meets the Green Knight, who turns out to be the lord of the castle. He spares Gawain's life, but Gawain gets nicked slightly because he didn't hand over the girdle to the lord even though he was supposed to. Gawain finds out that this whole thing was the idea of Morgan le Fay, Arthur's sister (god knows why). 

The most distinctive feature of Sir Gawain and the Green Knight is its "bob and wheel" form.  Example:

The boar makes for the man with a mighty bound

So that he and his hunter came headlong together

Where the water ran wildest - the worse for the beast,

For the man, when they first met, marked him with care,

Sights well the slot, slips in the blade,

Shove it home to the hilt, and the heart shattered,

And he falls in his fury and floats down the water,

                                         ill-sped.

          Hounds hasten by the score

          To maul him, hide and head;

          Men drag him in to shore

          And dogs pronounce him dead.

The author of Sir Gawain and the Green Knight is also sometimes known as "the Pearl poet," as he is also believed to be the author of the poems Pearl, Patience and Cleanness.