The story, illustrated by the author, is set in England during the Middle Ages, as the bubonic plague is sweeping across the country. Young Robin is sent away to become a knight like his father, but his dreams are endangered when he loses the use of his legs. A doctor reassures Robin that the weakness in his legs is not caused by the plague and the doctor is supposed to come and help him but does not. His parents are away, serving the king and queen during war, and the servants abandon the house, fearing the plague. Robin is saved by Brother Luke, a friar, who finds him and takes him to a monastery and cares for him.
Brother Luke teaches Robin how to swim and carve wood and make a harp, to be independent and build self-confidence, but Robin also learns patience and strength from the friar. The friar tells him that before overcoming a challenge you must first find "the door in the wall".
Robin's parents had planned for him to become a knight and to stay with Sir Peter de Lindsay to be a page first. John Go-in-the-Wynd, a minstrel, gives him a letter from Robin's father telling him and John Go-in-the-Wynd and Brother Luke to go to Lindsay. They get there after traveling for long hours, almost being robbed, and going on the wrong road for a time. When the castle of Peter de Lindsay is besieged by the Welsh and unable to send word for assistance, Robin swims the river, hobbles through enemy lines using his wooden crutches, disguised as a young shepherd, and alerts his messenger John Go-in-the-Wynd. John Go-in-the-Wynd sends for Sir Hugh's help, and they defeat the Welsh invaders. The king and his forces deliver the inhabitants of the castle. Robin is reunited with his parents and they assure him that they love him more for his brave spirit than for his physical prowess. He is then rewarded for his service to the crown.
The term "door in the wall" means if one keeps trying and never gives up, they'll find a way to break through and to succeed. A quote from the book reads, "If thou followeth a wall far enough, there must be a door in it."