The main characters of "The Quest for El Cid" are the kings of Leon-Castile, Sancho and Alphonso, who are caught up their dynastic succession struggle; Rodrigo Diaz de Vivar, known as 'El Cid' (from Sayyid in Arabic); the Ummayads; the Almoravids; the taifa city-states; the Reconquista; and the history of Rodrigo Diaz de Vivar as opposed to the legend of El Cid.

The Kingdom of Leon-Castile was the last European Christian stronghold in the north of Spain (former Roman Province of Hispania) by the 11th century. Most of Spain had been conquered, starting in 711, by an Islamic army of North African Berbers under the command of Arabs from the Ummayad Caliphate, which is based far across the Mediterranean in Damascus, Syria.

The Arab-Syrian Ummayads had been the rulers of Al-Andalus (as Muslim southern Spain was known in Arabic). The mighty Ummayad Caliphate had invaded and conquered vast swathes of the former Roman Empire and now ruled over a non-Muslim population that was majority Christian (along with large Zoroastrian and Jewish minorities). Internal disunity and revolt had broken Ummayad power in Al-Andalus and the numerous successor taifa city-states were now targeted for re-conquest by the Castilians and Leonese in order to free the subject Christians. The fundamentalist Almoravids of North Africa planned to use the crisis to invade and extend their power north by replacing the now-weakened taifa rulers.

The taifa city states of Andalusia were the fragments of the shattered Ummayad Caliphate in Spain. The internal breakdown of Ummayad rule occurred when weak dynasts were challenged and overthrown by powerful military generals, resulting in factional chaos. After Alphonso defeated Sancho for the throne of Leon-Castile, he succeeded in taking Toledo, and the panicked taifa states requested aid from the Almoravids of North Africa.

The Almoravids rose to power in the Maghreb (Muslim Northwest Africa) by promising their followers that a return to strict Islam would result in Allah's blessing, which would bring renewed military success and the legal plunder that accompanied it. They regarded the Ummayad and taifa rulers of Al-Andalus as corrupt and backsliding for engaging in non-Islamic practices like wine drinking, imposing non-Islamic taxes, and excessive toleration of Jews and Christians. They were invited to invade by the frightened taifa rulers, but the Almoravids had their own plans to betray and replace them.

Rodrigo Diaz de Vivar was a Castilian knight and member of the royal court (and the king's standard-bearer), who was exiled after supporting the losing side in a dynastic struggle. He made an effort to show loyalty to his sovereign, but his assembled military force was mercenary and fought for its own objectives, which could be complex given the fluid political situation in Spain. He conquered Valencia against massive numerical odds, and his name became legendary throughout the royal courts of Europe. His defeat of the Almoravids was a military watershed for European Christians and was used to inspire the forces of the Reconquista and the subsequent sense of Spanish nationhood.

The history of El Cid shows that early in his exile, he was as likely to fight as a mercenary for Muslims as for Christians. As Fletcher points out, there was hardly an idea of Spanish nationhood at this point, and complex strategic political allegiances often trumped religious ones. The times were changing, however, marking the beginning of the end of four centuries of defeat and humiliation for Spain's Christians. The collapse of the Ummayads, the fracturing of Al-Andalus into small Islamic taifa states, King Alphonso's re-conquest of Toledo, and the Roman Pope's intervention in the Castilian church emphasized that it was time for a harder line against the Moors. El Cid's widely celebrated re-conquest of the Mediterranean port city of Valencia showed that during the 11th century, the balance of power was finally shifting in favor of the European Christians.

The legend of El Cid is the transformation of the historical Castilian knight, Rodrigo Diaz de Vivar, into a unifying royalist symbol useful to future Spanish nationalists and the Reconquista. Fletcher works to distinguish the El Cid of history from the El Cid of later legend. Although the project of the Reconquista arguably pre-dates El Cid by a hundred years or so, Fletcher emphasizes the independent mercenary actions of El Cid in exile to temper and historicize our understanding of the legend of El Cid as used by royalists and nationalists. De Vivar lived in a period of transition, and the history and legend reflect this political complexity. The Castilian monarchy was having its own internal crisis and needed a heroic figure for loyalists to rally around, so the historical de Vivar was burnished in poetry and legend, and brought into the service of the Castilian state.