The Door in the Wall opens with the parents of the protagonist, Robin, having left to serve the king and queen. The servant caring for Robin has been taken by the plague, which is epidemic in London. Robin, ill, paralyzed, and alone, agonizes about the crippling illness which has stricken him following his parents’ departure. His main concern is that he will no longer be loved or accepted because of his inability to fulfill the roles and duties expected of the son of a nobleman in medieval times. He is rescued and nursed to health by Brother Luke, a Benedictine friar, who helps him restore his physical strength through swimming and who makes him a part of the active life of the monastery. The monastery is a busy hospice in which everyone is active. As soon as he is able, Robin is expected to enter into the life of the monastery. He finds that with something interesting to do and think about, the days pass quickly. During the six months he lives at the monastery, Robin learns reading, writing, history, music, gardening, handicrafts, and, of most importance, patience.
Upon directions received in a letter from Sir John, Robin’s father, Brother Luke and John-go-in-the-Wynd take Robin to Sir Peter’s castle in Lindsay. During the one-hundred-mile journey to this castle village in Shropshire, Robin has many adventures. He sleeps outdoors for the first time in his life, escapes from thieves, visits a country fair, and spends the night in a woodman’s cottage. Through Robin’s experiences on the journey, author Marguerite de Angeli is able to portray the life of the common people in contrast to that of the nobility. When Robin expresses concern that he will make a poor page, Sir Peter assures him that there are many ways to serve others and that each person has a rightful place in the world. Robin is taken into the life of the castle, learning many of the skills of a page and continuing his studies in music.
Shortly after John-go-in-the-Wynd leaves to visit his mother in a nearby village, the castle is besieged by the Welsh. When Robin finds that the food is dwindling and the well failing, he volunteers to go to John to enlist help, realizing that his crippled legs are an advantage in getting him through the Welsh lines without raising suspicion. After dressing in the clothes of a poor peasant and receiving a blessing from Brother Luke, Robin leaves the castle under the cover of fog. Following a dangerous descent down a cliff, he reaches the river, which he is able to cross because of all the practice he has had swimming. Although stopped by the Welsh, he is mistaken for a shepherd boy who has fallen in the river. Warmed by the enemy’s fire, he is sent on his way. John, learning of the danger to Lindsay, secures aid from a neighboring nobleman. He and Robin return in secret to Lindsay and wait in the church tower while the Welsh are routed and the castle is saved.
The story ends jubilantly on Christmas Eve. Robin’s mother and father come to the castle in the king’s company. They embrace their son, ignoring his crutches and crooked legs. Robin is reassured by his parents that his courage and spirit more than compensate for his crippled physical condition. The reader has the sense that the family is permanently reunited. Robin is knighted by the king for his courageous deeds, establishing his place in society.