In the North West of Spain, in the early years of the Fourteenth Century, the last progeny of two great houses have fallen in love, despite the wishes of the lady's father and amid the tumult of war that calls her suitor to battle.
Doña Beatriz Ossorio is the last surviving offspring of Don Alonso Ossorio, the lord of Arganza, and Doña Blanca de Balboa. And her lover, Don Álvaro Yáñez, lord of Bembibre, is also the last of his line. Although they have pledged their love to each other in the high chivalric manner, the lady's father has promised her hand in marriage to the count of Lemos, in the hope that great advantage will follow for himself and his family from an alliance with such a powerful nobleman and protégé of the Infante Don Juan, uncle of the King of Castile. After failing in an attempt to spirit Doña Beatriz away to a safe place, Don Álvaro goes to Castile to join battle with the royal forces besieging the castle of Tordehumos, but he is wounded and captured.
Her father's insistence on the arranged marriage affects Doña Beatriz's physical and mental state, and when she receives news that Don Álvaro is taken for dead, the lady agrees to her mother's dying request and consents to marry the count of Lemos. But Don Álvaro was imprisoned, not dead, and when he is released and learns of Doña Beatriz's marriage, contrary to her promise to him, the knight joins the Order of the Temple. He takes part in the defence of the Templar stronghold of Cornatel, in which the count of Lemos is killed. Although Doña Beatriz is now free to marry, Don Álvaro's vows to the Order deny him this freedom, pending a decision to be taken by the Church at the Council of Salamanca, which is judging the matter of the dissolution of the Templars.
The anxiety of their situation causes the lady's health to deteriorate further, so that when a dispensation arrives from the Pope allowing their union, the distress brings on a crisis and Doña Beatriz, on her deathbed, is married to Don Álvaro. Disconsolate, the lord of Bembibre settles his affairs and disappears from Spain, only returning near the time of his own death to watch over the burial place of his lady from the heights of Mount Aquiana.