WARNING: CONTAINS SPOILERS! The Seven Noble Knights of Lara is based on two prose texts from the thirteenth century, thought to be adapted from an oral tradition epic poem. The following is my summary of the events in the primary sources.
Ruy Blásquez, a valiant knight of Castile, marries Lambra, the noble cousin of the Count of Castile and invites all of Spain to attend, including his sister, Sancha, her husband, Gonzalo, and their seven sons, the seven noble knights of Lara. As a part of the celebrations, Ruy Blásquez sets up a lancing contest. Lambra's cousin Álvar Sánchez performs better than any other knight present, and she isn't afraid to let her feelings on this be known. Gonzalo, the youngest of the seven noble knights, decides to outdo him, and does, easily. Young Gonzalo and Álvar exchange angry words until Gonzalo becomes enraged and hits him, killing him instantly.
Doña Lambra tells her new husband to take revenge for the outrage, so he hits his young nephew, and young Gonzalo repays him by throwing his goshawk in Ruy Blásquez's face, breaking his nose. At that point, the Count of Castile steps in to calm the passions and prevent more deaths. The men agree to abide by his arbitration and leave the wedding festivities more or less satisfied.
Doña Lambra is not satisfied. Taking advantage of their husbands leaving on maneuvers, she invites Doña Sancha and her seven sons and their tutor to a hunting party. When the time is right, Lambra has her page soak a cucumber in blood. He obediently throws it at young Gonzalo and it hits him square in the chest. Gonzalo's brothers laugh at such a bizarre occurrence, but Gonzalo takes offense, saying that Lambra was proving she could fatally harm him. The law firmly on his side, he murders the page under Doña Lambra's protective skirts.
The González family leaves Doña Lambra on the estate and when the husbands return to their respective homes, each regroups. Ruy Blásquez proposes that his brother-in-law Don Gonzalo carry a letter to Córdoba, supposedly to collect a rich debt from Almanzor. Ruy Blásquez then takes his seven nephews to Almenar against the protestations of their tutor, who has read the auguries in the flights of birds and forecasts doom. An enormous army of Moorish mercenaries are waiting to ambush them in Almenar. Despite their valiant fighting and the aid of some of Ruy Blásquez’s troops, in the end, all seven brothers and the tutor are beheaded. The heads are sent to Almanzor in Córdoba. Meanwhile, Don Gonzalo, the father of the seven knights, has arrived in Córdoba only to find that the letter he carries is his death warrant. Almanzor does not behead him, but imprisons him in order to set up an investigation of the situation. Don Gonzalo is still in prison when they bring the heads to him for identification. His speech over his sons' remains are the most moving part of the poem.
Almanzor sends his sister to "comfort" Don Gonzalo, and by the time he returns to his home in Salas, the princess is pregnant with his last son. Don Gonzalo gives her half a ring by which to identify himself when he's ready to meet his father.
Twelve years later, Salas is in utter ruin, Don Gonzalo has cried so much that he's gone blind, and even the Count of Castile is having trouble with Ruy Blásquez, who marauds all over the land, taking Castilian castles for his own. In Córdoba, the princess's son, Mudarra is coming of age. He's been trained and shows extraordinary talent in every aspect a knight should. When a visiting potentate questions his legitimacy, he asks his mother who his father was. She tells him the story of the betrayal of the seven noble knights, and how his purpose in life is to find his father and set things right in Castile through bloody revenge.
When Mudarra arrives in Salas, Don Gonzalo realizes he can't recognize him as his son without incriminating himself with his wife, Doña Sancha, so he resists. Doña Sancha sees Mudarra as the opportunity they've been waiting for and demands that he recognize Mudarra. When he finally does, his sight is magically recovered and the broken ring fuses back together, never again to come apart.
Mudarra is adopted by Sancha, baptized as a Christian, and knighted by the Count of Castile in Burgos. Ruy Blásquez arrives to protest, and the count declares a temporary truce during which each party can put together his legal case. Ruy Blásquez instead flees with his soldiers and Mudarra and his father Don Gonzalo follow him. Each time they arrive where Ruy Blásquez was last sighted, he's already long gone. Finally, the enemies face off. After many formalities, Mudarra cuts Ruy Blásquez's arm so severely that he has to surrender. They bring him to his sister, Doña Sancha, who prepares to drink his blood to fulfill the dream she's had, but Don Gonzalo stops her. She decides her brother should be executed in a symbolic manner, so they tie him to a scaffold and throw stones and lances at him, in imitation of the sport the knights were taking part in during the ill-fated wedding.
Doña Lambra begs her cousin the Count of Castile for help, but is refused. Nevertheless, her blood ties with him make Mudarra unable to take an active revenge against her. She lives out her days on the run, with only one maid as a companion. She dies in the sierra, and when travelers pass her resting place, instead of saying a prayer for her soul, they say, "May she be damned!"