Viewers of HBO’s Rome may recall Irish actor Allen Leech playing Marcus Vipsanius Agrippa. In the TV series he is just entering his 20s and embarking on a dangerous adventure with his best friend, Gaius Octavian (played by Max Pirkis). Sadly, the show ended before fans could witness how they would change the world together.

His full but foreshortened life story is told in Robert Harris’s new novel, Agrippa. Now aged 50 and facing his own imminent death, Marcus Agrippa hurriedly writes his memoirs. He has much to reflect on, and the wisdom to make sense of it.

Agrippa begins with his birth in 64 or 63 BC. Roman texts tell us that Marcus had a “difficult birth” (aegre partus in Latin), which was conflated into the nickname Agrippa. Being born feet first was widely seen as so unlucky as to blight a man’s entire life. Yet Marcus bore his name proudly.

His luck turned when he befriended Octavius (the form used in the novel) just a few years before Julius Caesar’s assassination in 44 BC. For Harris’s hero, defying the odds is Agrippa’s motivation, the common thread through his life of derring-do and awesome achievements.

Agrippa marks Harris’s return to the world of ancient Rome. As in the Cicero Trilogy and Pompeii, politics and engineering feature prominently in Agrippa. In siding with Caesar’s legal heir and posthumously adopted son, Octavius, Agrippa becomes a revolutionary, enabling the overturn of the corrupt system that had suppressed the Roman people. In investing the spoils of civil and foreign wars in public baths and parks, Agrippa becomes a socially aware benefactor to ordinary Romans.

The Agrippa of history wrote a memoir of his time as commissioner of public works in Rome, a post he held from 33 BC. Its existence is known from facts about Agrippa’s aqueducts and fountains cited by Frontinus (who later took on the job) and Pliny the Elder (the Roman encyclopaedist). The autobiography itself is lost. Harris here recreates it. Imitating the style Agrippa was reputed to prefer, the language of the novel is unembellished and matter of fact. The result is a man of deeds rather than feelings.

The great mystery of Agrippa’s life is why he was so loyal to his friend when he could have seized supreme power for himself. Harris has an answer (I will not reveal the plot twist) and says he got the notion from FA Wright’s Marcus Agrippa: Organizer of Victory, published in 1937. While pure conjecture, it suits the needs of fiction.

Being Octavius’s best friend comes with both benefits and burdens. He puts his life at risk when fighting to remove his friend’s political enemies. He travels exhaustively around the world, quashing rebels in the provinces and providing administrative talent, to ensure peace in his name. He marries three times, divorcing twice at his friend’s request. The two boys of the last marriage to his friend’s daughter, Julia, are adopted in a symbolic sale so that Octavius might have heirs.

Yet, by collaborating with him, Agrippa enjoys fame and fortune that an unlucky man might not have achieved. From humble origins, Agrippa rises to become the second most powerful man in the Roman world.

Positioning the novel, the publisher asks rhetorically, “When it comes to power, does friendship exist at all?” Through civil wars and in their aftermath, Agrippa facilitated Octavius in becoming princeps, “First Man”. It gave him the right to convene and speak first at meetings of the Roman senate. Acclaimed by its members as Augustus (“Revered” or “Venerable”), he assumed national leadership, and with it control of the army.

While imperium (power) was centred on him, Agrippa shared the same powers and workload, lacking only his influence. He did so willingly and selflessly. He rejected triumphs, the coveted reward for victories in war, instead accepting a unique rostral crown and a blue pennant for his flagship. Yet after he died in 12 BC, Augustus made scant reference to Agrippa’s contributions in his Res Gestae, the official account he wrote of his deeds. Imperium was what mattered to Augustus. A pragmatist, Agrippa understood that. In death, he leaves everything to his friend.

I am delighted that Harris chose Agrippa as the subject of his latest novel. I must declare an interest: the author and I met last summer and discussed our shared fascination for the man. He invited me, as the author of Marcus Agrippa: Right-Hand Man of Caesar Augustus, to read the draft manuscript, and I checked it for errors.

Where I, as a historian, try to capture and interpret the past from known facts, the historical novelist has some licence for speculation, short cuts and rearrangements to make a compelling story. Applying his practised craft as storyteller, Harris brilliantly succeeds in bringing alive the man I recognise from the texts, statues and coins of Ancient Rome.

Lindsay Powell is a historian, writer and fellow of the Royal Historical Society