The novel opens in the year 1421. England, though engaged in warfare in France, enjoys peace and prosperity at home, but Scotland is in a state of anarchy. Its king, James I, the caged lion of the title, has been a prisoner in England since boyhood, though the king, Henry V, treats him as a friend. The chief character, Malcolm Stewart, is a timid, sickly, scholarly youth, who feels that the best way to protect his tenants and his sister is to give her in marriage to their soldierly cousin Patrick, and himself become a monk. This is found to be no solution. Lilias is placed in a nunnery, Patrick goes to fight the English in France, and Malcolm enters the service of his royal kinsman, James I. The tale is partly concerned with Malcolm's spiritual journey, partly with historical events, Henry V's last expedition to France and his death, and King James' return to Scotland. The historical characters are major not background figures. Most of the action takes place in France. While in England Malcolm improves in health and falls in love with Esclairmonde, who is vowed to a nun's life, and rejects him. He succumbs to the temptations of camp life in France, but repents and is able to save Esclairmonde from her wicked relatives. Returning to Scotland, he finds that his sister has been stolen from her convent, rescues her and restores her to Patrick who marries her. Malcolm gives up a life of peaceful scholarship to help King James bring order to Scotland. After the king's murder, he goes on pilgrimage to the Holy Land but dies on his return, having seen Esclairmonde again, and told her of the vision he has had of the two kings. The historical characters reveal themselves in their relationships with each other and with the fictitious ones.