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. When the great castle of Lindsay is attacked, Robin's determination and strength are tested. Robin finds that even with a disability, he can accomplish great things.