Isabel and her brother Alfonso live happily in Arévalo with their mother Isabel of Portugal, widow of King Juan II of Castile, along with her tutor Juan Chacón and his wife, Beatriz of Bobadilla. However, one bad day they are forcibly taken to the court of Segovia by order of their stepbrother, King Enrique IV, on the occasion of the pregnancy of his wife, Mrs. Juana of Avis. Isabel and Alfonso remain under the care of the queen, who despises and fears them at the same time because they constitute a threat to the future of the daughter she is waiting for.

Burgos, September 1464. The conspiracy led by Pacheco and Carrillo reaches its fullness with the Rebellion of the Nobles, where King Enrique is accused of kidnapping Isabel and Alfonso, of leaving power in the hands of Beltrán of the Cueva and not to put a stop to Moors and Jews.

Beatriz is shocked to learn that her father has given her hand in marriage without even consulting her. Meanwhile, the King decides to use another wedding to end the country's civil war: Isabel's.

The civil war in Castile comes to its most decisive point as the armies of Enrique and Alfonso face each other in Olmedo. In Segovia, the Queen prays for her husband and Isabel for her younger brother.

After their recent loss, Carrillo advices Isabel to continue the war and Chacón to seek peace. Enrique finds betrayal in a place he didn't expect. Chacón worries as Gonzalo and Isabel compare their feelings for each other.

Isabel and Enrique debate peace terms at Guisando. King Juan II of Aragon, seeking Castilian support against France, sends his emissary Peralta to negotiate the marriage of his heir Fernando - to Juan Pacheco's daughter.

Isabel debates her marriage options. Her supporters want her to marry Fernando, and Enrique, the King of France's brother. Pacheco and the King count as an advantage that the devout princess won't lie to them.

Fernando travels under disguise to meet Isabel in Valladolid, keeping an eye out for bandits and Pacheco's men. Isabel grows afraid as the time to sleep in the wedding bed comes near. Gonzalo can't hide his jealousy.

Isabel and Fernando marry in Valladolid with the support of several nobles and are acclaimed by the people. However they are cousins and the marriage needs a Papal bull to be valid.

Hearing good news from Rome, Enrique makes his new move by blockading Valladolid. With no food for the winter, the people's support for Isabel and Fernando vanishes. Fernando also grows sick of Carrillo's meddling.

Isabel and Fernando find refuge in Medina de Rioseco but their situation is dire. Pacheco's army is on the move, ready to punish their supporters. Fernando suspects of Carrillo, while news of her husband's infidelity drive Isabel mad.

King Juan convinces the new Pope to sign the bull authorizing Isabel and Fernando's marriage. Rome's envoy, Cardinal Borgia, also must propose a new Castilian cardinal - a position that Carrillo and Pedro Mendoza strive for.

Cabrera convinces the King to receive Isabel and Fernando in Segovia. Away from the Court, Pacheco offers Queen Juana his support while he plots to kidnap Isabel and her daughter.