Robin is a ten year old boy who is the son of a knight and it is therefore assumed that when he is older he will follow in his father's footsteps and become a knight himself. Finding himself alone in the big family house in the City of London, Robin's father having gone to war with the King and his mother assuming duties as lady-in-waiting to the Queen, Robin's legs inexplicably become misshapen and unable to bear his weight. He is lonely and frightened and because of this, not very nice to Dame Ellen, one of the household, who is coming down with symptoms of the plague, and at whom he hurls his bowl of oatmeal. Ellen is distraught and refuses to return, and the rest of the household are confined to their own family quarters because it is feared the plague is spreading.
It had been intended that Robin go to stay with Sir Peter Lindsay, his uncle, and learn how to be a page boy; however nobody was able to travel in or put of London until the plague had ceased spreading, so his trip was indefinitely delayed. Just as Robin was beginning to wonder of anyone would remember he was there, 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 had been generous with donations to support the work of the monks and so it had been decided that they would take care of him until he was 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 patient horse, Jenny, is waiting for them. Brother Luke leads Jenny with Robin on her back to the monastery where he settles Robin into his tiny quarters.
The monks care diligently for Robin, and he begins to get accustomed to his new surroundings. He enjoys listening to the day's comings and goings but is lonely as the monks have precious little spare time to spend with him. Brother Luke suggests he pass the time by whittling and Robin decides to whittle a boat out of soft pine. After less than two weeks he completes his project. It is the first time Robin has accomplished anything like this himself. Brother Luke suggests his next project should be a simple cross to hang over his bed. This will be whittled out of walnut wood.
As the weather gets warmer Brother Like decides Robin should spend more time outside and they finish the cross in the garden. He also promises to teach Robin to write. Robin dictates a letter to his father which rather bluntly explains his current predicament, then sends it off with John Go-In-The-Wynd, a messenger.
As June comes to an end so does the plague although still nobody travels much and many have died. Robin helps in the monastery kitchen and starts making a doll for one of the poor children served by the monastery. 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 allows him to play with other children on a more level playing field. Brother Luke tells him they are going to make crutches for Robin in exactly the same way he made the little pine cross and so strengthening his upper body becomes even more important.
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 terribly upset to hear Robin is unwell and unable to use his legs and hopes to keep the news from Robin's mother who will be unbearably worries. Robin is to head to Shropshore accompanied by Brother Luke and John Go-in-the-Wynd, and live with Sir Peter as planned. Plans for the journey began immediately, saddlebags were filled with food for the journey, and the trio set off with a beautiful sunrise to enjoy. This rapidly becomes a typically rainy English day. The journey is arduous and they take a wrong turn, and unable to reach the White Swan by nightfall they sleep outside . The following day they find an inn called the White Hart and although they do not like the look of the clientele or the goodwife there they decide to stay overnight. Robin overhears two ruffians plotting to rob them as they sleep. He wakes Luke and John and warns them; they throw Brother Luke's cloak from the window and secure it so that they can climb down it and escape without the men seeing them. The men see what they have done and give chase but so not catch them.
Finding a granary on the outskirts of the village they sleep what is left of the night there and set off again in the morning. 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 more hilly - and also more rainy. After spending a night at the home of a woodcutter and his wife, they see Lindsay, 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. He need not have worried, though, because the family greet him warmly and decide that he will perform only tasks within his physical capabilities. Robin loves the family and the castle. He also forms an amazing bond with one of Sir Peter's dogs, D'Ath, who follows him everywhere and seems to view Robin as his master. The Welsh have crossed the border and taken the town, and are hammering at the castle gate trying to take it over. The castle keep is lowered and for the moment the Welsh are shut out but food is scarce and the well is almost dry; a lean harvest has not helped build a store of food necessary for a drive like this. Robin manages to come up with a plan; he will leave the castle and flee to the home of John-Go-In-the-Wund's mother nearby and ask John to get Sir Hugh, Sir Peter's cousin, to liberate the castle by attacking the Welsh who will be surprised by the attack.
Although Sir Peter and Sir Hugh are not speaking, Sir Hugh wants the Welsh pushed back just as much as Sir Peter does. Robin dresses in rags so that if he is stopped he will be thought a simple shepherd boy. His descent from the castle is treacherous; he must climb down a deep ravine which takes a long time but he eventually finds himself on the ground by the edge of the river. He fastens his crutches to his back and then plunges into the icy river, almost too cold to move at first, but then feeling warmer and powerful as he swims to the other side with strong strokes. He feels the shallower water on the other side and scales the river bank. Suddenly a voice demands to know who he is; Robin says he is a shepherd boy. He mumbles something about going to the village and thinking him simple-minded the Welsh let him go on. About an hour later he is through the forest and manages to locate the cottage he is seeking Calling John's name, he is relieved to see John running towards him. Robin explains the situation then as John sets out to take the news to Sir Hugh, Robin rests. Finding that he has slept most of the day, Eobin learns from John that Sir Hugh's expert lancers are already on the way to Lindsay. He and Robin then leave the cottage for the castle, taking a secret route through the south east of the forest, and entering 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 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 they will wait for an hour after curfew. At the agreed hour, they ring the church bells which is a signal for Sir Hugh's army to shoot a torrent of arrows towards the Welsh army. The arrows pick off 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 beaten. Robin and John-Go-in-the-Wynd receive a hero's welcome at the castle and Sir Peter hails Robin as a true son of his father. John is called forward and given land, sheep and fishing rights to be his family's in perpetuity.
Winter becomes colder and the days seem short as it becomes dark early. Soon it will be the Feast of Christmas. On Christmas Eve, Robin is at his usual position at the top of the keep, when Adam Bowyer 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, Robin's mother, Lady Maude, 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 happy to see his son and oblivious to his withered legs and the crutches. The King's party retires for a while before the Christmas Eve celebrations. 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. He 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 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 and on waking Brother Luke tells him he is safe with his loved ones, and that he found his door in his wall.