David Reubeni claimed to command the army of a mighty Jewish kingdom located in Arabia. His diary describes his sixteenth-century travels through Africa, Asia, and Europe as he struggled to forge political alliances and obtain weapons. Reubeni obtained the patronage of a number of prominent Italian Jews. As he traveled through Portugal, he also gained the support of many of its conversos. This introduction places Reubeni's mission in the context of Renaissance-era imperialism, speculates on his motivations, and evaluates his legacy. It also discusses what happened to Reubeni after his diary ends and the events that led to his execution in 1538.

David Reubeni leaves his home in the Arabian Jewish kingdom on a mission on behalf of his brother, King Joseph, and crosses the Red Sea into Africa. Disguised as a Muslim sayyid, he travels until he reaches the land of Amara Dunqas, the founder of the Funj Sultanate. He travels throughout the area with Amara until their relations sour and Reubeni is forced to leave and make his way to Egypt.

Still disguised as a Muslim sayyid, David Reubeni enters Cairo. He fails to gain the support of its Jews and is cheated by his Muslim host, with the result that he loses the treasure that he had brought along. Still disguised as a Muslim, he leaves for Palestine and visits the Cave of the Patriarchs and the Dome of the Rock. He apprises some Jews in Gaza of his secret mission and they provide him with funds to leave for Venice.

David Reubeni arrives in Venice and raises funds for his trip to Rome. In Rome, he meets Cardinal Egidio di Viterbo, who arranges an audience for him with Pope Clement VII. Reubeni also builds ties with many prominent Italian Jews. Most importantly, the da Pisa family becomes his patron and provides servants and funds for his growing entourage. Reubeni is delayed in Rome for a year while the pope decides whether or not to help him. The pope eventually agrees to provide him with letters of introduction to the King of Portugal and Prester John. Reubeni attempts to travel to Portugal but he is delayed by the Portuguese ambassador, Dom Miguel, who does everything in his power to thwart him. Dom Miguel is eventually recalled and Reubeni receives a document of safe passage to Portugal.

David Reubeni sails to Portugal. His relations with João III, the King of Portugal, are initially successful, with the king promising to provide him with ships, weapons, and weapons experts. Reubeni, however, attracts the attention of numerous conversos who are enthusiastic about his mission. He is accused of attempting to help the conversos return to Judaism. Matters come to a head when a converso high court judge, Diogo Pires (Solomon Molkho), circumcises himself and returns to Judaism. Reubeni is blamed for these events and the king ends his negotiations with him. King João allows Reubeni to return to the pope, providing that he does not take any conversos with him.

As a result of a storm, Reubeni's ship is forced to anchor at Almería in the territory of the Holy Roman Emperor, Charles V. Despite his letters from the King of Portugal and the pope, he is arrested because Jews have been banned from Spain since 1492. Eventually, the emperor grants him safe conduct but an inquisitor from Murcia ignores this and has him briefly arrested. It is upon his release that the diary ends. Although Reubeni has minimal interactions with conversos in Spain, he does report some warm interactions with the Moriscos, Muslims who had been forcibly converted to Christianity.

Reubeni's companion, Solomon Cohen, briefly continues the diary. Reubeni and Solomon Cohen were shipwrecked off the coast of France. There they were arrested in 1527 by the "Lord of Clermont," likely Francis William of Clerman, the papal legate in Avignon. Reubeni's belongings were confiscated and he was imprisoned for two years. He was released only after some local Jews paid a heavy ransom.