Miguel Domínguez, known as "El Cautivo" is the last of a class of knights, the brown knights, who controlled a vast territory known as La Extremadura del Duero by Christians, or Al-Sarrasín in Muslim chronicles, and not controlled by the Christian kings of the north nor the Muslims of the south. It is the border between two worlds, the Christian and the Muslim ones, and as a border territory it is vast, dangerous and violent, in which the only law that prevails is that imposed by the brown knights, keepers of large herds of cattle from their raids in Muslim land and which has become their way of life ... and death. Christian kings will use them in their fight against the Muslims to extend and expand their kingdoms, but they cannot control them so they will continue fighting on their own, defending their ancestral way of life, liberty and independence. The novel tells the life and passing of one of these gentlemen, Miguel Domínguez, El Cautivo, the last of these brown knights. He tells us, in a pleasant and entertaining way, his childhood in which he gets military training to defend his people’s flocks, his friendship with one of those young people who will serve him as a guide and training companion, the awakening of the young man to life, to the pleasures it provides, and that leads to a dissipated and licentious life his father will put a halt to by sending him to Extremo, where men become tough and forget their licentious youth games because their lives depend on it. And Miguel Domínguez, El Cautivo can check this when he sees many of the knights from the city die on the border. That is the fate for the "brown men" and the city will lose many of its men at the hands of Muslim armies. But this is their way of life and how they are shaped as men, above all defending their freedom and independence, because they are defending their way of life, which for hundreds of years has been followed by their people. Miguel Domínguez, El Cautivo, will be caught and live in captivity for a few years during which we will see what the life at a Muslim border city is like. When he is released, he will have face his responsibilities, debts incurred by his family, but love will also emerge, a forbidden love, that even a society as free as that of the border cannot tolerate. Yet he will not hesitate to dive into it with the same spirit of freedom that guides his life, and face the consequences. But this society he defends is a border society which  attracts all kinds of unscrupulous people, criminals, looking for a place to get rich or get rid of the chains imposed by feudal lords in the northern kingdoms. Duels, intrigue, hatred and interests come together in it, while telling us about life in Salamanca, a border town which the Christian kings, once they feel stronger and more powerful, do not hesitate to try to submit. And the brown knights, in a final attempt to survive, to defend their independence and freedom, do not hesitate to confront their own king, because their lives, their own lives depend on it, even if they perish in the attempt and have to suffer the betrayal of their own kind. Miguel Domínguez, El Cautivo will continue to tell us, through his life, his fascinating history and that of his people. A song of freedom of an entire people and that of a man, their last representative.