- The soul begins explaining the history of Rome. He talks about how Emperor Constantine committed the crime of moving the capital of the Roman Empire away from Rome to Byzantium. By so doing, Constantine turned the Empire against Heaven and against centuries of good leadership.
- After hundreds of years, the soul says, Rome came to be ruled by me, Justinian, who was known for reforming the Roman laws.
- Then he was converted to Christianity by the words of Pope Agapetus. Through God's inspiration, Justinian created the Codex Justinianus, a huge listing of all the Roman laws that did not include contradictions, complications, or pagan ideals. This allowed the Empire some peace.
- But he wants Dante to see the hypocrisy of the situation: some people pretended to support the Holy Roman Church, but really opposed it.
- Justinian goes back to the very beginning of Roman history, starting with the death of Pallas (whom Turnus killed) and the founding of Rome, through its seven monarch. He then chronicles the era of the Roman Republic and its successful rebuffing of Hannibal, (the great general of the Carthaginians).
- Justinian then details the ascent of Julius Caesar (which occurred around the middle of the 1st century B.C.) and all of his conquests: the story of Cleopatra in Egypt, Caesar's rivalry and civil war against Pompey, and the treachery of Brutus and Cassius, who assassinated Caesar.
- The new emperor Augustus took revenge on Caesar's murderers and finally brought peace to the Empire.
- But, Justinian narrates, the most important is yet to come.
- During the reign of Tiberius, Christ was crucified, finally reconciling man and God (for Adam's original sin). God took vengeance on the Jews (for Christ's death) by having the emperor Titus destroy Jerusalem. Then Charlemagne helped legitimate Christianity in the Roman Empire.
- Now, Justinian says, Dante can understand why he holds the Guelphs and the Ghibellines (the hypocrites he mentioned earlier) in such contempt. The Guelphs openly opposed the Roman Empire, while the Ghibellines treacherously took the emblem of the Empire (the Eagle) and made it their own.
- Then he warns Charles of Anjou, leader of the Guelphs, to beware of Rome.
- Justinian talks about the souls inhabiting this planet—Mercury. We are all righteous spirits, he claims. Everyone on this planet is motivated by his or her desire for fame. Justinian recognizes now that to value fame so much is to love wrongly.
- But he and his fellows rejoice in what God has given them because it is just. And they're happy to be a part of "differing voices," which "render sweet harmony among these spheres."
- Justinian starts praising a guy named Romeo of Villaneuve, who was a poor pilgrim whose virtue got him appointed to the position of minister to Count Raymond Berenger of Provence. Romeo married each of Berenger's four daughters into a royal family so that they eventually became queens. Romeo was a favorite of the court, but then jealous people planted a rumor that Romeo was swindling money from his boss, so they got Berenger to accuse Romeo formally. Though innocent, Romeo was offended and so he renounced his position, took what few possessions he originally had, and left—leaving Berenger begging for him to come back.
- If the world had known Romeo's true heart, Justinian says, they would have appreciated him even more.