Final answer:
Sir Gawain agrees to let the Green Knight return the axe blow in a year and a day at the Green Chapel after striking him, setting up the story's main adventure.
Step-by-step explanation:
In the Arthurian tale, after Sir Gawain strikes the Green Knight with the axe, he agrees to a daunting challenge. Gawain will allow the Green Knight to return the blow in a year and a day at the Green Chapel. This agreement sets the stage for the central narrative of courage, honour, and chivalry within the Sir Gawain and the Green Knight story, a classic of Middle English literature.