The First Man in Rome is the story of Marius, a Roman patrician living during the last days of the Roman Republic. Told largely in letters written between its primary characters, the story follows Marius's political career from his advantageous marriage to his suppression of an armed revolt in the streets of Rome. The book ends with Marius an old, unwell man at the end of his career, his goodwill among the people largely dried up.
The main plot of the novel is generally concerned with the rise of Marius, his marriage to Julia, his success in replacing Metellus as general in charge of the Numidian theatre of war, his defeat of King Jugurtha of Numidia, his re-organization of the Roman Army system, his unprecedented consecutive consulships, his defeat of a massive invasion of German tribes (the Teutones, the Cimbri and the Marcomanni/Cherusci/Tigurini), and the details of his relationship with his subordinate and close friend Sulla.
However, although Marius can be considered the protagonist, Sulla occasionally becomes the central figure of the narrative; there are several lengthy sections dealing with his plot to murder the two wealthy women with whom he lives, his use of the newfound wealth in establishing himself politically, his homosexual relationship with the Greek child-actor Metrobius, and his marriage to the (fictitious) younger daughter of 'Julius Caesar Grandfather', Julilla. McCullough explains that, while it is certainly known Sulla's first wife was a Julia, it is not known to which branch of the Julii she belonged, but she was certainly a relation of Marius's better-known Julian wife, hence the decision to assign her the role in the novel of a younger sister.
A third storyline is focused on the figures of Marcus Livius Drusus and his sister Livia Drusa who both feature more prominently in The Grass Crown : and their own growing friendship with the Servilius Caepio family resulting in a double marriage, which proves disastrous when Quintus Servilius Caepio Senior is not only accused of embezzling more gold than there was in the Roman Treasury, but also is responsible for Rome's most disastrous military defeat for generations– a defeat which so ruins the credibility of the conservative leaders of the Senate that it lets Marius into power far earlier than he expected, and for a longer time.
Much of the narrative is also told in the form of letters between the protagonists– Marius, Sulla, Old Caesar and frequently their friend, Publius Rutilius Rufus – himself a man somewhat torn in allegiance: conservative by instinct, but partisan of Marius by friendship.
The novel closes with Marius's sixth consulship, in which he proves not to be as adept politically as he is militarily: and the tribune whose help he needs, Lucius Appuleius Saturninus, has an agenda of his own, leading to an armed insurrection which Marius himself has to put down. To cap it all, he also suffers a minor stroke during the summer, although he makes a full recovery. Tarred by association, his political career seems over: but after fighting many battles together, there is some reason for Marius and Sulla to hope that Rome will have peace for a few years.