Robin, son of Sir John de Bureford, has been told his whole life that one day he, like his father, would learn the ways of knighthood. Yet one day, while his parents are away, Robin is struck by a illness that leaves him unable to use his legs. Left alone in plague-ridden London, Robin is taken to the monastery of St. Mark's by a kind monk named Brother Luke.

Robin is saddened by the fact that, as a cripple, he cannot become a knight like his father. "Thou has only to follow the wall far enough," Brother Luke tells Robin, "and there will be a door in it." While recovering at the monastery, Robin discovers that door. He becomes skilled at woodcarving and swimming, and is taught to use his mind as well as his hands. He is also given a more valuable lesson in patience.

When he is well, Robin is sent to the castle of Sir Peter de Lindsay, where he was to learn the ways of knighthood, but now he will simply serve as a page. Robin wonders what sort of page he can be with crutches and, consequently, without free hands for service, but when the castle falls under attack, it is Robin who can save the lord and his people.