It describes how Sir Gawain , a knight of King Arthur 's Round Table , accepts a challenge from a mysterious `` Green Knight '' who challenges any knight to strike him with his axe if he will take a return blow in a year and a day .
He challenges any knight to strike him with his axe , if he will take a return blow in a year and a day . Sir Gawain , a knight of King Arthur 's Round Table , accepts the challenge .