Robin is a 10-year-old boy who is the son of two noble people: Sir John de Bureford and Lady Maude. It is assumed that when Robin is older, he will follow in his father's footsteps and become a knight. Robin has been left alone in his London home after his father goes to fight in the war against the Scots and his mother assumes duties as lady-in-waiting to the Queen. While his parents are away, Robin's legs inexplicably become misshapen and unable to bear his weight. He is lonely and frightened, confined to his bed for days on end; around him, more and more of the household servants come down with the plague. Frustrated with his condition, Robin takes out his anger on his caretaker, Dame Ellen, at whom he hurls his bowl of oatmeal. It is revealed that Dame Ellen also has the plague; she is no longer to care for Robin.
Robin was expected to go and stay with Sir Peter de Lindsay, his uncle, and learn how to be a page boy. However, nobody is able to travel in or out of London until the plague is contained, so Robin's trip has been indefinitely delayed. Just as Robin is beginning to wonder if anyone will remember him, he sees a monk standing in the doorway of his bedroom. The monk introduces himself as Brother Luke, a new friar at St. Mark's Monastery. Robin's father was generous with donations to support the work of the monks; so, it has been decided that Brother Luke would take care of the boy until he is able to make the trip up north. As Robin cannot walk, Brother Luke hoists him onto his back and takes him outside to where his horse, Jenny, is waiting for them. Brother Luke takes Robin to the monastery where he settles him into his tiny quarters.
The monks care diligently for Robin, and Robin begins to get accustomed to his new surroundings. He enjoys listening to the day's comings and goings but feels lonely, as the monks have little spare time to spend with him, overwhelmed with their daily duties. Brother Luke suggests that Robin pass the time by teaching himself how to whittle. Robin decides to whittle a boat out of soft pine. In less than two weeks, he completes his project. It is the first time Robin has accomplished anything so creative, and he feels happy with himself. Brother Luke suggests that his next project should be a simple cross to hang over his bed; he will whittle it out of walnut wood.
As the weather gets warmer, Brother Luke decides that Robin should spend more time outside, so they finish the cross in the garden. He also promises to teach Robin to write. Robin dictates a letter to his father, which bluntly explains his ailment. Brother Luke sends the letter off with John Go-In-The-Wynd, a messenger.
As June comes to an end, so does the plague, although people are still cautious with their travels and many have died. Robin helps in the monastery kitchen and starts carving a doll for one of the poor children served by St. Marks. He also learns to fish and is happy to be spending more time outdoors. Brother Luke encourages him to go in the water and swim; not only does this strengthen his upper body, but it also allows him to finally play with other boys on a more level playing field. Brother Luke tells Robin they are going to carve wooden crutches for him to start walking again, and so strengthening his upper body becomes even more of a priority.
Towards the end of September, John-go-in-the-Wynd returns with a message from Robin's father. Robin is excited that, thanks to his study with Brother Luke, he is able to read it for himself. His father is upset to hear Robin is unwell and unable to use his legs and hopes to keep it from Robin's mother, who would be worried to hear such news. Sir John tells Robin to head to Shropshire with the company of Brother Luke and John Go-in-the-Wynd in order to live with Sir Peter, as originally planned.
Arrangements for the journey begin immediately. Saddlebags are filled with food, and the trio set off with a beautiful sunrise to enjoy. The journey is rather arduous. At one point, they take a wrong turn and are unable to find the White Swan Inn by nightfall, so instead, they sleep outside. The following day, they find an inn called the White Hart; although they do not like the look of the place, they decide to stay there overnight anyway. Robin overhears two ruffians plotting to rob them as they sleep. He wakes Luke and John to warn them; they throw Brother Luke's cloak from the window and secure it so that they can climb down and escape without the thieves seeing them. The ruffians see what they have done and try to chase them, to no avail. The trio runs to a granary on the outskirts of the village they sleep for the remaining hours of the night.
After four days of travel, they reach Oxford, where they are given hospitality at St. John's College. They also happen upon a fair where they watch a Punch and Judy show, which Robin loves. That afternoon, they reach an abbey where they stay the night, and the countryside changes, becoming hillier. After spending a night at the home of a woodcutter and his wife, they see the town of Lindsay in the distance, with its castle on the top of the hill. Robin becomes nervous about Sir Peter's reaction to his withered legs and his difficulty in performing some of the traditional page boy tasks. His worries are unfounded, though, because Peter and his family greet him warmly and assure Robin that he will only perform tasks within his physical capabilities. Robin loves the family and the castle. He also forms a close bond with one of Sir Peter's dogs, D'Ath, who follows him around everywhere.
One very foggy day in late October, the neighboring Welsh army attempts to take over Lindsay. The Welsh have crossed the border and are hammering at the castle gate. Sir Peter orders the men to action and the women to guard themselves in the keep. The Welsh are temporarily kept out, but the castle's food and water supplies are dwindling and some of the men worry about how they will sustain themselves. At once, Robin comes up with an ingenious plan: he will escape the castle, disguised as a poor shepherd boy, and flee to the home of John-go-in-the-Wynd's mother to ask John for help from Sir Hugh, a cousin of Sir Peter.
Robin takes off. His descent from the castle is treacherous: he must climb down a deep ravine and swim through an icy river with his crutches on his back. After making it across, he is stopped by a voice demanding to know who he is. Through the fog, Robin can see it is a Welshman; Robin plays himself off as a shepherd and is able to make it past the soldier. About an hour later, he is through the forest and manages to locate the cottage of John's mother.
Reunited with John, Robin explains the situation and immediately sets off to get help from the army of Sir Hugh. Robin, exhausted, is served a meal by John's mother and then sleeps for many hours. Upon waking, Robin learns that John has already sent Sir Hugh's expert lancers to Lindsay. He and Robin then leave the cottage for the castle, taking a secret route through the forest. They plan to enter the town through the shoemaker's cottage, which is built into the city wall. John plays a tune on his little harp, and the shoemaker replies with a little jig played on the bagpipes. Ascertaining that all is safe, a chair is let down from a high window for Robin and he is hauled aloft into the upper room of the house. They quickly leave again through the garden and head for the church. The sacristan escorts them to the bell tower, where it has been agreed that they will wait for an hour after curfew.
At the agreed-upon hour, they ring the church bells, which is a signal for Sir Hugh's army to shoot a torrent of arrows at the Welsh army. The arrows disturb the guards at the town wall and Sir Hugh's men enter the town gate, taking back the town. The plan is a success, and the Welsh are defeated. The whole town of Lindsay celebrates, and Robin and John-Go-in-the-Wynd receive a hero's welcome at the castle. Sir Peter hails Robin as the true son of his father. John is called forward and given land, sheep, and fishing rights.
It becomes winter, and the Feast of Christmas is approaching. On Christmas Eve, Robin is at his usual position at the top of the keep, when he notices a company of knights and men approaching. At the head is the King, and, at his side, Robin recognizes his father. In the middle of the large company of riders is the Queen, and Robin's mother, Lady Maude, is beside her. The Scottish wars are over! Robin decides to wait for the party inside with Sir Peter to make sure that his parents are able to recognize him. Robin's mother throws her arms around him, holding him close; his father is happy to see his son and is oblivious to his withered legs and the crutches. The King's party retires for a while before the Christmas Eve celebration begins.
Robin enters the hall with Brother Luke at his side. He kneels before the King who has heard of his service and announces that he is the true son of a noble father. The entire kingdom is proud of him. The King drops a jeweled collar over his shoulders and there is deafening cheering. Robin then sings a Christmas carol for the King and, as he finishes, the room erupts with cheers for Sir Robin. His father tells him that his courage and spirit shine so bright a light that he cannot see that his legs are misshapen. Lady Maude tells Robin that they are all going to return home to London together and that Brother Luke shall be offered the position of his tutor. Robin falls asleep; upon his waking, Brother Luke tells him that he is safe with his loved ones, and that he found his door in his wall.
